Use docker:latest container for job execution

Run the job in a docker:latest container which has Docker
pre-installed, avoiding the need to install it manually.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

.forgejo/workflows/docker-build.yml

@@ -8,6 +8,7 @@ on:
 jobs:
   build-and-push:
     runs-on: ubuntu-latest
+    container: docker:latest
 
     steps:
       - name: Checkout repository