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/dependabot.yml

@@ -0,0 +1,19 @@
+version: 2
+updates:
+  # Maintain dependencies for GitHub Actions
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "weekly"
+    labels:
+      - "dependencies"
+      - "github-actions"
+
+  # Maintain dependencies for Docker
+  - package-ecosystem: "docker"
+    directory: "/"
+    schedule:
+      interval: "weekly"
+    labels:
+      - "dependencies"
+      - "docker"