Allow building PR images on demand (#340)

Triggered when the label is set in a PR.
Helps to test changes in PRs.

Co-authored-by: crapStone <codeberg@crapstone.dev>
Reviewed-on: https://codeberg.org/Codeberg/pages-server/pulls/340
Reviewed-by: crapStone <codeberg@crapstone.dev>
Co-authored-by: pat-s <patrick.schratz@gmail.com>
Co-committed-by: pat-s <patrick.schratz@gmail.com>

.woodpecker/build.yml

@@ -114,6 +114,23 @@ steps:
       - event: ['push']
         branch: ${CI_REPO_DEFAULT_BRANCH}
 
+  'Publish PR image':
+    image: woodpeckerci/plugin-docker-buildx:3.2.1
+    depends_on: test
+    settings:
+      registry: codeberg.org
+      dockerfile: Dockerfile
+      platforms: linux/amd64
+      repo: codeberg.org/codeberg/pages-server
+      tags: next
+      username:
+        from_secret: bot_user
+      password:
+        from_secret: bot_token
+    when:
+      evaluate: 'CI_COMMIT_PULL_REQUEST_LABELS contains "build_pr_image"'
+      event: pull_request
+
   docker-tag:
     depends_on: vendor
     image: woodpeckerci/plugin-docker-buildx:3.2.1