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>

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,36 @@
+## Description
+
+<!-- Describe your changes in detail -->
+
+## Type of Change
+
+<!-- Mark with an `x` all that apply -->
+
+- [ ] Bug fix (non-breaking change which fixes an issue)
+- [ ] New feature (non-breaking change which adds functionality)
+- [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
+- [ ] Documentation update
+- [ ] CI/CD update
+
+## Testing
+
+<!-- Describe the tests you ran to verify your changes -->
+
+- [ ] Built Docker image locally
+- [ ] Tested container startup
+- [ ] Tested repository cloning
+- [ ] Tested Happy Coder integration
+- [ ] Tested VNC web interface
+
+## Checklist
+
+- [ ] My code follows the style guidelines of this project
+- [ ] I have performed a self-review of my own code
+- [ ] I have commented my code, particularly in hard-to-understand areas
+- [ ] I have made corresponding changes to the documentation
+- [ ] My changes generate no new warnings
+- [ ] I have tested that the Docker image builds successfully
+
+## Screenshots (if applicable)
+
+<!-- Add screenshots to help explain your changes -->