ci: prebuild the debian toolchain as a container image

.dockerignore

@@ -0,0 +1,4 @@
+# The image is a toolchain only -- the dockerfile COPYs nothing, and the
+# scripts arrive via actions/checkout at run time. Keep the build context empty.
+*
+!dockerfile

.gitea/workflows/build-image.yaml

@@ -0,0 +1,67 @@
+---
+name: Build Autoupdater Image
+
+concurrency:
+  group: build-image
+  cancel-in-progress: false
+
+on:
+  schedule:
+    # Monthly, to pick up bookworm point releases and security updates;
+    # :latest is overwritten each time.
+    - cron: "0 3 1 * *"
+  push:
+    branches:
+      - main
+    paths:
+      - dockerfile
+      - .dockerignore
+      - .gitea/workflows/build-image.yaml
+  workflow_dispatch:
+
+defaults:
+  run:
+    shell: bash
+
+jobs:
+  build:
+    # No Debian needed here -- this only drives docker. The *image* is Debian.
+    runs-on: ubuntu-latest
+
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Log in to the Gitea container registry
+        env:
+          GIT_HOST: ${{ vars.GIT_HOST || 'git.bjphoster.com' }}
+          REGISTRY_USER: ${{ vars.REGISTRY_USER }}
+          GIT_TOKEN: ${{ secrets.GIT_TOKEN }}
+        run: |
+          set -euo pipefail
+          # NB: this is the Gitea *username*, not vars.GIT_NAME -- that one is
+          # the display name used for git commit authorship and contains spaces.
+          printf '%s' "${GIT_TOKEN}" \
+            | docker login "${GIT_HOST}" \
+                --username "${REGISTRY_USER}" \
+                --password-stdin
+
+      - name: Build and push
+        env:
+          GIT_HOST: ${{ vars.GIT_HOST || 'git.bjphoster.com' }}
+        run: |
+          set -euo pipefail
+          # The repo is `.autoupdate`, but OCI names cannot start with a dot.
+          image="${GIT_HOST}/deb/autoupdate"
+
+          # --pull so a monthly rebuild actually picks up a new base image
+          # rather than reusing a cached one.
+          docker build --pull --tag "${image}:latest" .
+          docker push "${image}:latest"
+
+          echo "pushed ${image}:latest"
+
+      - name: Log out
+        if: always()
+        env:
+          GIT_HOST: ${{ vars.GIT_HOST || 'git.bjphoster.com' }}
+        run: docker logout "${GIT_HOST}" || true

dockerfile

@@ -0,0 +1,46 @@
+# Build environment for the Debian autoupdater.
+#
+# Rebuilt monthly by .gitea/workflows/build-image.yaml and pushed as :latest, so
+# the nightly package jobs start with the toolchain already in place instead
+# of each running a full apt transaction first.
+#
+# bookworm on purpose: a package built against the older dpkg/debhelper installs
+# on bookworm and on everything newer, never the other way round.
+FROM debian:bookworm-slim
+
+LABEL org.opencontainers.image.title="deb-autoupdate"
+LABEL org.opencontainers.image.description="Debian build environment for the Debian package autoupdater"
+LABEL org.opencontainers.image.source="https://git.bjphoster.com/deb/.autoupdate"
+
+ARG DEBIAN_FRONTEND=noninteractive
+
+# nodejs          actions/checkout is a JavaScript action; the runner executes it
+#                 with `node` from inside this image, and debian-slim ships none.
+# dpkg-dev        dpkg-buildpackage, dpkg-parsechangelog
+# debhelper       dh, for debian/rules
+# build-essential dpkg-checkbuilddeps implicitly requires it, even for arch:all
+# fakeroot        for packages without Rules-Requires-Root: no
+# devscripts      dch and friends, for working on a package by hand in here
+# lintian         advisory lint of the built package
+RUN apt-get update \
+ && apt-get install -y --no-install-recommends \
+      nodejs \
+      git \
+      curl \
+      jq \
+      ca-certificates \
+      dpkg-dev \
+      debhelper \
+      build-essential \
+      fakeroot \
+      devscripts \
+      lintian \
+ && rm -rf /var/lib/apt/lists/*
+
+# The nightly job drops to this user rather than building as root (CI
+# containers are root by default). Baked in here rather than created per run.
+RUN useradd --create-home --shell /bin/bash builder
+
+# Sanity check: fail the image build rather than the nightly job if the
+# toolchain ever stops working in this environment.
+RUN runuser -u builder -- bash -c 'dpkg-buildpackage --version | head -n1 && lintian --version && command -v dpkg-parsechangelog node'