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/update.yaml

@@ -37,8 +37,14 @@ jobs:
     # emits an empty .SRCINFO, which the AUR then rejects -- that is what broke
     # reflex-appimage, nethlink-appimage and pman-helper. regen_srcinfo() in
     # common.sh fails loudly if this ever regresses.
+    #
+    # Prebuilt by build-image.yaml (see dockerfile) so the toolchain is not
+    # reinstalled in all six jobs every night.
     container:
-      image: archlinux:base-devel
+      image: git.bjphoster.com/aur/autoupdate:latest
+      credentials:
+        username: ${{ vars.REGISTRY_USER }}
+        password: ${{ secrets.GIT_TOKEN }}
 
     strategy:
       # One broken package must not cancel the rest of the matrix.
@@ -53,15 +59,10 @@ jobs:
           - tsparams
 
     steps:
-      # Must come BEFORE actions/checkout. That action is JavaScript, and the
-      # archlinux image ships no node, so checkout dies with
-      # `exec: "node": executable file not found in $PATH`. A `run:` step needs
-      # no checkout, so the toolchain can be installed first.
-      - name: Bootstrap container
-        run: |
-          pacman -Syu --noconfirm --needed \
-            nodejs git openssh jq curl pacman-contrib namcap fuse2 python
-
+      # No bootstrap step: node, git, openssh, jq, curl and the makepkg
+      # toolchain all come from the prebuilt image above. node in particular
+      # has to be present before this action runs, since actions/checkout is
+      # JavaScript and the runner executes it with `node` from the image.
       - uses: actions/checkout@v4
 
       - name: Configure git and SSH
@@ -91,11 +92,11 @@ jobs:
           EOF
           chmod 600 ~/.ssh/config
 
-      # makepkg refuses to run as root, and CI containers are root by default.
-      - name: Create build user
+      # The `builder` user is baked into the image; this only hands it the
+      # workspace and the credentials written by the previous step.
+      - name: Hand over to the build user
         run: |
           set -euo pipefail
-          useradd --create-home --shell /bin/bash builder
           chown -R builder:builder "${GITHUB_WORKSPACE}"
           cp -r /root/.ssh /root/.gitconfig /root/.git-credentials /home/builder/ 2>/dev/null || true
           chown -R builder:builder /home/builder