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

.gitea/workflows/ci-images.yml

@@ -0,0 +1,94 @@
+name: CI Images
+
+# Publishes the per-PHP-version images that ci.yml and release.yml run inside
+# (#187). Nothing else consumes them, so this workflow is the only place the
+# registry path is written down.
+#
+# Triggers:
+#   - the Dockerfile or this workflow changing on main, so an edit ships;
+#   - the same paths on a pull request, which builds but does not push, so a
+#     broken Dockerfile is caught before it reaches main;
+#   - workflow_dispatch, to rebuild on demand;
+#   - weekly, so PHP patch releases and Alpine security updates land without
+#     anyone remembering to ask.
+
+on:
+  push:
+    branches:
+      - main
+    paths:
+      - '.gitea/ci/Dockerfile'
+      - '.gitea/workflows/ci-images.yml'
+  pull_request:
+    paths:
+      - '.gitea/ci/Dockerfile'
+      - '.gitea/workflows/ci-images.yml'
+  schedule:
+    - cron: '17 4 * * 1'
+  workflow_dispatch:
+
+env:
+  # The instance ROOT_URL host — the container registry lives on the same host.
+  REGISTRY: git.unsupervised.ca
+  IMAGE: unsupervised/ci-php
+
+jobs:
+  build:
+    name: Build CI image (PHP ${{ matrix.php }})
+    runs-on: ubuntu-latest
+    strategy:
+      # One version failing should not hide whether the others built.
+      fail-fast: false
+      matrix:
+        # Keep in step with the test matrix in ci.yml.
+        php:
+          - '8.1'
+          - '8.2'
+          - '8.3'
+          - '8.5'
+    steps:
+      - uses: actions/checkout@v4
+
+      # The images are built natively, so they carry the runner's
+      # architecture only. Fine while every runner is arm64; if a runner of a
+      # different architecture ever joins the pool it will overwrite these
+      # tags with its own arch and the others will fail to pull.
+      - name: Check Docker is available
+        run: |
+          if ! docker info >/dev/null 2>&1; then
+            echo "No usable Docker daemon in the job container." >&2
+            echo "act_runner needs container.docker_host set (or left empty to autodetect)." >&2
+            exit 1
+          fi
+          docker version --format 'client {{.Client.Version}} / server {{.Server.Version}} / arch {{.Server.Arch}}'
+
+      # secrets.GITHUB_TOKEN is the Actions task token and can write packages
+      # for the repository owner. REGISTRY_TOKEN is an escape hatch: set it to
+      # a PAT with package:write if the task token is ever refused.
+      - name: Log in to the container registry
+        if: github.event_name != 'pull_request'
+        run: |
+          echo "${{ secrets.REGISTRY_TOKEN || secrets.GITHUB_TOKEN }}" \
+            | docker login "${REGISTRY}" -u "${{ vars.REGISTRY_USER || github.actor }}" --password-stdin
+
+      - name: Build
+        run: |
+          docker build \
+            --pull \
+            --build-arg "PHP_VERSION=${{ matrix.php }}" \
+            --tag "${REGISTRY}/${IMAGE}:${{ matrix.php }}" \
+            --file .gitea/ci/Dockerfile \
+            .gitea/ci
+
+      # Pull requests build only — the tags on the registry are what the other
+      # workflows run inside, so only main and a manual dispatch move them.
+      - name: Push
+        if: github.event_name != 'pull_request'
+        run: |
+          image="${REGISTRY}/${IMAGE}:${{ matrix.php }}"
+          docker push "${image}"
+          echo "Published ${image}"
+
+      - name: Log out
+        if: always() && github.event_name != 'pull_request'
+        run: docker logout "${REGISTRY}" || true