feat: Automated install, Docker deployment, and CI Docker job (#42)

- debian/control: add Pre-Depends and Depends on postgresql-16, argon2
- debian/postinst: idempotent automation for PostgreSQL setup,
  DB/user creation, migration tracking, admin password generation,
  config write, and service enable/start
- Dockerfile: multi-stage build (Rust + frontend + slim runtime)
- docker/entrypoint.sh: first-run DB wait, migrations, admin password
- docker-compose.yml: split db/app architecture with healthcheck
- .env.example: template for DB_PASSWORD and TAG
- .dockerignore: exclude build artifacts from Docker context
- .github/workflows/ci.yml: add Docker job for multi-arch
  (amd64/arm64) GHCR push on tag releases with layer caching
- .gitignore: add .env entry

.github/workflows/ci.yml

@@ -12,6 +12,7 @@ env:
 
 permissions:
   contents: write
+  packages: write
 
 jobs:
   rust-format:
@@ -128,3 +129,46 @@ jobs:
         with:
           body: ${{ steps.release_notes.outputs.notes }}
           files: linux-patch-manager_*.deb
+
+  docker:
+    name: Docker Build & Push
+    needs: [rust-format, clippy, rust-test, security-audit, frontend-lint]
+    if: startsWith(github.ref, 'refs/tags/v')
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v3
+
+      - name: Set up QEMU
+        uses: docker/setup-qemu-action@v3
+
+      - name: Log in to GitHub Container Registry
+        uses: docker/login-action@v3
+        with:
+          registry: ghcr.io
+          username: ${{ github.actor }}
+          password: ${{ secrets.GITHUB_TOKEN }}
+
+      - name: Extract Docker metadata
+        id: meta
+        uses: docker/metadata-action@v5
+        with:
+          images: ghcr.io/draco-lunaris/linux-patch-manager
+          tags: |
+            type=semver,pattern={{version}}
+            type=semver,pattern={{major}}.{{minor}}
+            type=semver,pattern={{major}}
+            type=sha
+
+      - name: Build and push Docker image
+        uses: docker/build-push-action@v6
+        with:
+          context: .
+          platforms: linux/amd64,linux/arm64
+          push: true
+          tags: ${{ steps.meta.outputs.tags }}
+          labels: ${{ steps.meta.outputs.labels }}
+          cache-from: type=gha
+          cache-to: type=gha,mode=max