Publish prebuilt CI images to the Gitea container registry

setup-php installs PHP 8.3+ from apt on these arm64 runners: a ~145s floor
against ~35s for 8.1/8.2, with a tail that twice ran past the step timeout
and failed the run (#178). Caching the .debs softened it without removing
the apt step, and 8.5 has the same problem.

Add a per-version CI image built on php:<version>-cli-alpine and a workflow
that publishes it to git.unsupervised.ca/unsupervised/ci-php:<version>. The
org is public, so the packages pull anonymously.

The image carries bash and nodejs because act_runner runs JavaScript actions
inside the job container, GNU coreutils/grep/sed because the workflow scripts
use `tac` and `grep --include`, and curl/jq/git/zip for release.yml and
bin/build-zip.sh. Composer 2 and the intl and zip extensions round it out.

Nothing consumes the images yet — ci.yml and release.yml switch over in a
follow-up, because a job cannot run in an image that has not been published.

Part of #187

Co-Authored-By: Claude Opus 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01D9acV1mHktGAb1uyvNmrR2

docs/ci.md

@@ -0,0 +1,70 @@
+# CI images
+
+CI and release jobs do not install PHP. They run inside prebuilt images
+published to the Gitea container registry:
+
+```
+git.unsupervised.ca/unsupervised/ci-php:8.1
+git.unsupervised.ca/unsupervised/ci-php:8.2
+git.unsupervised.ca/unsupervised/ci-php:8.3
+git.unsupervised.ca/unsupervised/ci-php:8.5
+```
+
+The `Unsupervised` org is public, so the packages pull anonymously — jobs need
+no registry credentials to use them.
+
+## Why
+
+`shivammathur/setup-php` installs PHP 8.3+ from apt/the ondrej PPA on these
+arm64 runners. That was a ~145s floor against ~35s for 8.1 and 8.2, with a
+tail that twice ran past the step timeout and failed the run outright
+(#178). Caching the `.deb`s helped, but the apt step itself remained, and PHP
+8.5 has the same shape of problem. Pulling a ~120MB image from a registry
+inside the cluster replaces the whole thing (#187).
+
+## What is in the image
+
+`.gitea/ci/Dockerfile` builds on `php:<version>-cli-alpine` and adds:
+
+- **`bash` and `nodejs`** — act_runner runs JavaScript actions
+  (`actions/checkout`, `actions/cache`, `actions/upload-artifact`) *inside*
+  the job container and shells `run:` steps through bash. Without these, the
+  first step of every job fails.
+- **`coreutils`, `gawk`, `grep`, `sed`** — GNU versions, because the workflow
+  scripts use `tac` and `grep --include`, which busybox does not provide.
+- **`curl`, `jq`, `git`, `zip`, `unzip`** — used by `release.yml` and
+  `bin/build-zip.sh`.
+- **`intl` and `zip` PHP extensions**, plus Composer 2. `mbstring` is already
+  compiled into the official images.
+
+## Publishing
+
+`.gitea/workflows/ci-images.yml` builds and pushes them. It runs when the
+Dockerfile changes on `main`, weekly (so PHP patch releases and Alpine
+security updates land on their own), and on `workflow_dispatch`. On a pull
+request it builds without pushing, so a broken Dockerfile is caught before it
+reaches `main`.
+
+## Adding or dropping a PHP version
+
+1. Add the version to the `php` matrix in `.gitea/workflows/ci-images.yml`.
+2. Merge to `main`, or dispatch the workflow, and wait for the tag to appear.
+3. Add the version to the `test` matrix in `.gitea/workflows/ci.yml`.
+
+Steps 2 and 3 cannot be one commit: a job cannot run in an image that has not
+been published yet.
+
+## Architecture
+
+The images are built natively on whichever runner picks the job, so they carry
+that runner's architecture only. Every runner in the pool is arm64 today. If
+one of a different architecture ever joins, it will overwrite these tags with
+its own arch and the rest will fail to pull — at which point the build needs
+`docker buildx` and a multi-arch manifest.
+
+## If a push is refused
+
+The build authenticates with `secrets.GITHUB_TOKEN`, the Actions task token.
+If the registry ever refuses it, create a personal access token with
+`package:write`, store it as the `REGISTRY_TOKEN` secret, and optionally set
+the `REGISTRY_USER` variable — the workflow prefers both when present.