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

CHANGELOG.md

@@ -11,6 +11,8 @@ When a `v*` tag is pushed, `.gitea/workflows/release.yml` publishes the matching
 the plugin to the next patch version and adds a fresh section here for it. Record
 each change under the current top section as you work.
 
+## [1.5.5]
+
 ## [1.5.4]
 
 ### Fixed

unsupervised-schedular.php

@@ -3,7 +3,7 @@
  * Plugin Name: Unsupervised Scheduler
  * Plugin URI:  https://git.unsupervised.ca/Unsupervised/unsupervised-scheduler
  * Description: Instructor/student lesson scheduling for WordPress.
- * Version:     1.5.4
+ * Version:     1.5.5
  * Requires at least: 6.2
  * Requires PHP: 8.1
  * Author:      Unsupervised
@@ -21,7 +21,7 @@ if (! defined('ABSPATH')) {
     exit;
 }
 
-define('USC_VERSION', '1.5.4');
+define('USC_VERSION', '1.5.5');
 define('USC_PLUGIN_FILE', __FILE__);
 define('USC_PLUGIN_DIR', plugin_dir_path(__FILE__));
 define('USC_PLUGIN_URL', plugin_dir_url(__FILE__));