Switch container builds to docker runner (Phase 1 ratcheting)

Test that the containerized runner can build containers.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

.forgejo/workflows/build-container.yaml

@@ -15,7 +15,7 @@ on:
 
 jobs:
   build:
-    runs-on: ubuntu-latest
+    runs-on: docker
     steps:
       - name: Parse tag
         id: parse

.github/actionlint.yaml

@@ -1,5 +1,3 @@
 self-hosted-runner:
   labels:
-    - docker-builder
-    - ubuntu-latest
-    - ubuntu-22.04
+    - docker