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0015. Container image and its publication

• Status: accepted

• Date: 2026-11-07

Context

Housecairn is installed by people who run a home server, mostly with Docker Compose, on x86 machines and on ARM boards. They expect one image that works with SQLite out of the box and with PostgreSQL if they want it (ADR 0008, 0012), and they will not read a hardening guide. The image must also be verifiable: anyone should be able to check that a published image was built by this repository’s workflow from a given commit, and what it contains.

Decision

Base image

python:3.13.<patch>-slim-trixie, the official Python image on Debian 13 slim, pinned by digest with its tag on the same line; both stages use the same digest so the virtual environment built in the first runs in the second. Debian rather than Alpine: Debian’s glibc takes the manylinux wheels, the format every Python package with compiled code publishes first, for amd64 and arm64. Today every locked runtime dependency with compiled code (cffi, cryptography, psycopg-binary, charset-normalizer, pyyaml, rpds-py) also publishes musllinux wheels for both, but that is not guaranteed for the next dependency, and a missing wheel means a compiler and build headers in the image build. musl also differs from glibc in ways that surface at run time (DNS resolution, locales, thread stack size), which the tests on the runner, a glibc system, would not see. A distroless base was not chosen: the entry point needs a Python interpreter matching the one of the virtual environment, and the slim image’s small set of Debian packages is scanned and patched by Debian.

Build

Two stages:

1. build: uv (copied from its official image, same version as the CI) installs the locked dependencies with uv sync --locked --no-dev --extra postgres --no-install-project into /app/.venv, with bytecode compiled. The project is not installed as a package: its source is on PYTHONPATH (/app/src), so no unlocked build backend is downloaded. Then design tokens (build_design_tokens) and translations (compilemessages, GNU gettext installed in this stage only) are generated from their sources, and static files are collected with CompressedManifestStaticFilesStorage (hashed names, compressed copies).

2. final: the same base, without its pip (unused at run time, and the source of most scanner findings), a system user and group housecairn, UID/GID 10001, and /app copied from the build stage, owned by root, hence read-only for the application.

.dockerignore excludes everything, then lets in only pyproject.toml, uv.lock, LICENSE, NOTICE, manage.py, design/ and src/: no .git, no test, no local data or .env file can reach the image. The OCI labels source, licenses (AGPL-3.0-only), version and revision are set from build arguments.

Runtime dependencies added

ADR 0001 is amended by this record:

Package	Why
gunicorn	Production WSGI server, pure Python, MIT. Started with its control socket disabled (it would need a writable path) and its worker heartbeat files in /dev/shm.
whitenoise	Serves the collected static files from the application, with long-lived immutable cache headers for hashed names and precompressed copies. Keeps the image a single process tree, without a web server in front only for static files. MIT.

Run time

• Single writable place. Everything written at run time goes to /data: SQLite database, uploaded files (/data/media), the secret key, backups. The rest of the file system works mounted read-only (--read-only), with /tmp as a tmpfs. Python does not write bytecode at run time (PYTHONDONTWRITEBYTECODE; it is compiled at build time).

• Entry point python -m housecairn.entrypoint (src/housecairn/entrypoint.py, tested in tests/test_entrypoint.py):

  • creates /data/media and fails with an explanation if /data is not writable by the application user;

  • sets the process umask to 077: files in /data are readable by the application user only;

  • uses SECRET_KEY from the environment if set, otherwise /data/secret_key, created on first start with 50 random bytes (secrets.token_urlsafe), mode 600, then reused;

  • applies the migrations (migrate --noinput), then starts gunicorn and the django-q2 worker (qcluster) and supervises them: if either stops, the other gets SIGTERM and the container exits with a non-zero status (128 + signal number if it was killed), so the container’s restart policy applies; SIGTERM or SIGINT stops both, waits up to HOUSECAIRN_STOP_TIMEOUT seconds (20 by default) then kills what remains, and exits 0. As PID 1, it also collects orphaned processes. A tiny supervisor in the entry point was preferred to supervisord or s6: two processes, no configuration file, and behaviour covered by tests.

  • healthcheck, backup (SQLite online backup API into /data/backups/) and manage <command> subcommands.

• No account is created. The first administrator comes from the existing sign-in paths (ADR 0007): the first OpenID Connect sign-in, optionally pinned to HOUSECAIRN_BOOTSTRAP_ADMIN_EMAIL, or manage createsuperuser.

• Health check. HEALTHCHECK runs entrypoint healthcheck, which requests /healthz on the loopback with a host name taken from ALLOWED_HOSTS; the endpoint answers only {"status": "ok"} and needs no authentication (ADR 0012).

• Proxy headers. gunicorn trusts X-Forwarded-Proto from the addresses of HOUSECAIRN_FORWARDED_ALLOW_IPS only (--forwarded-allow-ips, default the loopback), and from nobody unless HOUSECAIRN_BEHIND_TLS_PROXY is true; Django applies the same list to every forwarding header (ADR 0012). Trusting any peer (*) was the first choice, since a proxy’s address in a container network is not known in advance; it let a client that reaches the port directly pass for HTTPS and choose its address for the sign-in limits. The example compose.yaml therefore fixes its network’s range, so that the address a proxy on the host comes from, the gateway, is known.

Workflow image.yml

Job	Trigger	Permissions	What it does
Container image	pull request, push to main, tag v*	contents: read	Builds the image for amd64 and runs .github/scripts/image-smoke.sh on it, with SQLite then with PostgreSQL: started with --read-only --cap-drop ALL --security-opt no-new-privileges, /tmp in memory and an empty volume; healthy; /, the sign-in page and a hashed static file answer; user 10001; writes outside /data and /tmp refused; secret key mode 600; clean stop on SIGTERM; restart on the same volume keeps the database and the key and applies no migration; killing the task worker stops the container. Validates compose.yaml. Scans the image with Grype: an inventory of every finding (job summary and grype-inventory artifact), then the fixable ones for code scanning and the check.
Container image (code scanning)	same, except pull requests from forks and Dependabot	security-events: write	Uploads the Grype SARIF of the previous job to code scanning; that file holds only vulnerabilities with a fix available. Runs no code of the pull request.
Container image (multi-arch build)	pull request	contents: read	Builds linux/amd64 and linux/arm64 with provenance and SBOM, as the publication does, without pushing.
Publish image	push to main, tag v*, after Container image passed	contents: read, packages: write, id-token: write, attestations: write	Builds and pushes both platforms to GHCR, then attests provenance and SBOMs.

Multi-architecture: QEMU emulation on the amd64 runner (the tonistiigi/binfmt image, pinned by digest, run with docker run) and a docker-container BuildKit builder (moby/buildkit, pinned by digest). The image only installs prebuilt wheels and runs Python, so emulation is fast enough, and one docker buildx build --platform linux/amd64,linux/arm64 produces the multi-platform image and its attestations in one step. Native arm64 runners were not chosen: they would need one job per architecture, an artifact per digest and a merge job, for a build of a few minutes.

Tags (scripts/image_tags.py, tested): on main, edge and sha-<7 hex>; on a tag vX.Y.Z, X.Y.Z, X.Y, latest and sha-<7 hex>; on a pre-release tag vX.Y.Z-<pre>, X.Y.Z-<pre> and sha-<7 hex> only. Nothing is published from a pull request or another branch.

Provenance and SBOM. BuildKit generates SLSA provenance (--provenance=mode=max) and an SPDX SBOM per platform (--attest type=sbom, with the docker/buildkit-syft-scanner image pinned by digest), stored with the image. The workflow then signs them with GitHub artifact attestations (Sigstore, keyless, through the job’s OIDC token): build provenance on the multi-platform index digest, and the SPDX SBOM of each platform, extracted with docker buildx imagetools inspect, on that platform’s digest. Attestations are also pushed to the registry. They are checked with gh attestation verify (docs/install.md). Artifact storage records are not created (create-storage-record: false), so the job needs no artifact-metadata permission.

The card that specified this lot asked for actions/attest-build-provenance and actions/attest-sbom. Both are now wrappers of actions/attest, which their own documentation asks to migrate to (attest-sbom’s README announces its deprecation); this workflow uses actions/attest directly, with the same inputs.

Vulnerability scan: Grype (Anchore), installed from its GitHub release archive whose SHA-256, taken from the release’s checksums file, is pinned in the workflow. Not Trivy: its release binary and its GitHub actions were compromised in March 2026 (CVE-2026-33634, GHSA-69fq-xp46-6x23); Grype does the same job with an equivalent database. The Container image job fails if the image has a vulnerability of high or critical severity with a fix available (--only-fixed --fail-on high). Code scanning (category container-image) receives the same set, every severity but only what has a fix (see the amendment of 2026-11-07 below). A vulnerability without a fix does not block a release: there is nothing to update to.

Amendment, 2026-11-07: code scanning receives fixable vulnerabilities only. The first scans put 189 alerts in the Security tab, 181 of them without any fix published by Debian (glibc, SQLite, util-linux, systemd, tar, coreutils). The project cannot act on those: no dependency update, no code change or base image digest removes them, and they buried the few alerts that can be resolved. The SARIF now comes from grype --only-fixed (an option of the pinned Grype 0.121.0: « ignore matches for vulnerabilities that are not fixed »), the same filter as the blocking check, so the tab lists what the project can fix. The job proves it on every run: the JSON of the same scan must contain no match without a fix, and its match count must equal the number of SARIF results. The blocking command itself is unchanged.

Nothing is dropped. A separate step, which cannot fail the job (continue-on-error), scans again without the filter and writes to the job summary the counts by severity and by package and the list of fixable vulnerabilities; the full result (grype-all.json and a table) is the grype-inventory artifact of the run, kept 14 days. The published image’s SBOM, attested on every release, lets anyone rescan it later.

An empty Security tab therefore does not mean « no known CVE »: the image carries vulnerabilities that Debian has not fixed yet, and the base image may carry more tomorrow. They are listed in the inventory of each run. Alerts already open without a fix close by themselves at the first analysis uploaded after this change reaches main, because the new SARIF no longer contains them.

Versions and cooldown. The tools pinned in the workflow (Grype, BuildKit, binfmt, the SBOM scanner) were chosen among releases public for at least seven days, as the Dependabot cooldown of ADR 0014 does for dependencies. The Python base image digest is the latest of its tag: those rebuilds carry Debian security updates. Dependabot updates the FROM lines (docker ecosystem) and the PostgreSQL image of compose.yaml (docker-compose ecosystem). The pinned tools of image.yml (environment variables) and the PostgreSQL image of ci.yml and image-smoke.sh are updated by hand, together.

Third-party actions

None. image.yml uses only actions published by GitHub: actions/checkout, actions/upload-artifact v7.0.2 (the SARIF and the inventory), actions/download-artifact v8.0.2, actions/attest v4.2.2 and github/codeql-action/upload-sarif. QEMU, BuildKit and the registry login go through the docker command line rather than Docker’s actions.

Checks to add to the main ruleset

Once each has passed once on GitHub:

• Container image

• Container image (multi-arch build)

Not required: Container image (code scanning) (skipped for forks and Dependabot), Publish image (never runs on a pull request).

Consequences

• One image, ghcr.io/housecairn/housecairn, for both databases and both architectures; an instance’s state is the /data volume and nothing else.

• A host directory mounted on /data must belong to 10001:10001.

• What only happens after a merge to main cannot be tested on a pull request: the push to GHCR, the attestations and the first publication of the package (its visibility and its link to the repository are set once, by the maintainer, after the first push).

• A fixable high or critical vulnerability in the base image or a dependency blocks pull requests until it is updated. Dependabot proposes the update for a dependency; for a Debian package of the base image, see the amendment of 2026-11-08 below.

• The X.Y and latest tags move to the most recently published release, even if it is a patch of an older line.

Move to Python 3.13 (2026-11-07)

The image started on python:3.12.16-slim-trixie. The scanner (Grype) reported eight CVEs against the Python 3.12.16 interpreter, with fixed versions in 3.13 or later. The CPython CNA records list:

• fixed in 3.13: CVE-2025-12781 (3.13.10), CVE-2026-3446 and CVE-2026-3479 (3.13.13), CVE-2026-6019 (3.13.14), CVE-2025-15366 (3.13.15);

• not fixed in any 3.13 release: CVE-2025-15367 (fixed in 3.15.0a6), CVE-2026-103995 and CVE-2026-116566 (fixed in 3.16.0).

The project therefore moved to Python 3.13, which Django 5.2 LTS supports. The base image is now python:3.13.18-slim-trixie, pinned by digest as before, in both stages. The CI runs the tests, the type checks and the Django checks on 3.13, so that what is tested is what is shipped. requires-python is >=3.13,<3.14 and uv.lock was regenerated for it (same package versions).

The Dependabot rule is unchanged in substance: it ignores minor and major updates of the python image (a jump to 3.14 is a migration lot) and keeps the 3.13.x patch releases. The three CVEs above stay reported until a Python line that fixes them is adopted; they are accepted until then.

Amendment, 2026-11-08: Debian security fixes ahead of the base image

Debian published DSA-6613-1 for zlib (CVE-2026-76844 and CVE-2026-85091, high, plus CVE-2026-27171, medium), fixed in 1:1.3.dfsg+really1.3.1-1+deb13u1 in trixie-security. The pinned python:3.13.18-slim-trixie still had 1:1.3.dfsg+really1.3.1-1+b1, so the Container image check failed on every pull request and on main, and no image could be published. No newer image of the 3.13 line carried the fix: the tag had been rebuilt the same day, but its amd64 and arm64 manifests were the same as those of the pinned digest (only riscv64 had changed), and their Debian sources point to a snapshot of 2026-11-04, without the fix.

The final stage therefore upgrades the named packages, and only those, from the base image’s own trixie-security source (apt-get install --only-upgrade), then checks that the installed version is at least the fixed one, so that a mirror still serving the old package fails the build instead of shipping it. The step lists each package with its CVEs and fixed version.

The compromise: the image no longer depends only on the base image digest and the lock file. Two builds of the same commit can get different versions of a listed package if Debian publishes a newer security update in between; the version can only go up, and never below the fixed one. Pinning the exact version was not chosen, for the reason the build stage already gives: a pinned Debian version leaves the mirrors at its next security update and breaks every build. Installing from a fixed snapshot.debian.org timestamp would be reproducible but makes every build depend on that service, slower and rate limited. A general apt-get upgrade was not chosen either: it would change every package of the base image, not just the ones a finding names. The blocking scan is unchanged (--only-fixed --fail-on high); nothing is ignored.

The step is temporary per package: once Dependabot moves the FROM lines to a base image digest that carries the fix, the package leaves the list, and the step goes away when the list is empty.

Dependabot could not have proposed this change. Its security updates follow the GitHub Advisory Database, which does not cover the Debian packages of an image, and no base image digest with the fix existed. When the Python image is rebuilt with it, the weekly schedule and the 7-day cooldown of the docker ecosystem delay the digest update by up to two weeks. A possible setting, not applied here, is to exempt the python base image from that cooldown (cooldown: exclude: ["python"]) and to check it daily, keeping the cooldown for the other images.

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