feat: Add GitHub Actions for automated Docker image builds

Add comprehensive CI/CD pipeline for building and publishing Docker images
to GitHub Container Registry (ghcr.io).

Components:
- build-and-push.yaml: Main workflow for building and pushing images
  - Triggers on push to main, tags, PRs, and manual dispatch
  - Multi-tag strategy (latest, semver, sha, branch)
  - Docker buildx with layer caching
  - Automatic GHCR authentication
  - Platform: linux/amd64

- release.yaml: Automated GitHub releases on version tags
  - Auto-generates release notes from commits
  - Creates GitHub release with Docker pull command

- dependabot.yml: Automated dependency updates
  - Weekly updates for GitHub Actions
  - Weekly updates for Docker base images

- PULL_REQUEST_TEMPLATE.md: PR template with testing checklist

Updated README:
- Add build status badge
- Update image pull instructions
- Reference automated builds

Image tagging strategy:
- main branch → :latest, :main, :main-{sha}
- v1.2.3 tag → :v1.2.3, :1.2, :1, :latest
- PR → :pr-{number}

Generated with [Claude Code](https://claude.ai/code)
via [Happy](https://happy.engineering)

Co-Authored-By: Claude <noreply@anthropic.com>
Co-Authored-By: Happy <yesreply@happy.engineering>

README.md

@@ -1,5 +1,7 @@
 # Antigravity Dev Container
 
+![Build and Push](https://github.com/cpfarhood/devcontainer/actions/workflows/build-and-push.yaml/badge.svg)
+
 A containerized development environment with GUI access, featuring:
 - **Antigravity** (VSCode/Cloud IDE) via web browser
 - **Happy Coder** - AI-powered development assistant
@@ -35,11 +37,22 @@ A containerized development environment with GUI access, featuring:
 
 ## Quick Start
 
-### 1. Build the Image
+### 1. Get the Image
+
+The image is automatically built and published to GitHub Container Registry on every push to main.
 
 ```bash
-docker build -t ghcr.io/cpfarhood/antigravity:latest .
-docker push ghcr.io/cpfarhood/antigravity:latest
+# Pull the latest image
+docker pull ghcr.io/cpfarhood/devcontainer:latest
+
+# Or pull a specific version
+docker pull ghcr.io/cpfarhood/devcontainer:v1.0.0
+```
+
+**Building locally (optional):**
+```bash
+docker build -t ghcr.io/cpfarhood/devcontainer:latest .
+docker push ghcr.io/cpfarhood/devcontainer:latest
 ```
 
 ### 2. Configure Secrets