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.

.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