ci: prebuild the debian toolchain as a container image

dockerfile

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+# 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'