Use full Ubuntu image with Docker pre-installed

Based on Zen multi-model consensus analysis:

Root cause: act_runner v0.2.13+3 maps ubuntu-latest to bare
ubuntu:22.04 which lacks Docker CLI. Newer runners use full images.

Solution: Override job container to use ghcr.io/catthehacker/ubuntu:full-22.04
which includes Docker, Buildx, Node.js, and standard CI tools.

This is the recommended approach from act_runner maintainers for
GitHub Actions compatibility.

Consensus from Gemini-2.5-pro and o3 models.

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

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

.gitea/workflows/build-deploy.yml

@@ -8,6 +8,8 @@ on:
 jobs:
   docker:
     runs-on: ubuntu-latest
+    container:
+      image: ghcr.io/catthehacker/ubuntu:full-22.04
 
     steps:
       - name: Checkout code