ci: prebuild the Arch environment as a container image

All six nightly jobs ran the same pacman transaction before doing any work.
The toolchain now lives in an image published to the Gitea registry as
git.bjphoster.com/aur/autoupdate:latest, rebuilt monthly since Arch is rolling
and a month-old image is a month behind.

The build job itself runs on plain ubuntu-latest -- it only drives docker. Only
the image is Arch.

Based on archlinux:base rather than base-devel: the autoupdater parses
PKGBUILDs and never compiles, so the toolchain is dead weight, 822 MB against
1.48 GB. nodejs is included because actions/checkout is a JavaScript action the
runner executes with node from inside the image.

Slimming to base initially dropped diffutils, and apply_pkgrel_rule used
diff -q. A missing binary makes the `if` fail, which reads as "changed", so
pkgrel ratcheted upward on every run -- the same silent-wrong-default class as
the empty .SRCINFO this rewrite was about, and invisible to the host test run.
The comparison is now a shell string equality with no external dependency, and
is verified against an image with no diff at all. require_tools() additionally
fails on startup for any missing binary rather than midway through.

.gitea/workflows/build-image.yaml

@@ -0,0 +1,67 @@
+---
+name: Build Autoupdater Image
+
+concurrency:
+  group: build-image
+  cancel-in-progress: false
+
+on:
+  schedule:
+    # Monthly. Arch is a rolling release, so a month-old image is a month
+    # behind on packages; :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 Arch needed here -- this only drives docker. The *image* is Arch.
+    runs-on: ubuntu-latest
+
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Log in to the Gitea container registry
+        env:
+          GIT_HOST: ${{ vars.GIT_HOST }}
+          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: ${{ vars.GIT_HOST }}
+        run: |
+          set -euo pipefail
+          # The repo is `.autoupdate`, but OCI names cannot start with a dot.
+          image="${GIT_HOST}/aur/autoupdate"
+
+          # --pull so a monthly rebuild actually picks up a new archlinux base
+          # 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: ${{ vars.GIT_HOST }}
+        run: docker logout "${GIT_HOST}" || true