feat(ci): push Docker image to ghcr.io on main

On main branch: build and push with sha-<commit> and latest tags.
On PR: build only (verify image is buildable).
Images: ghcr.io/nantian-gw/nantian-controlplane

Author: mahmut-Abi

.github/workflows/docker.yml

@@ -0,0 +1,50 @@
+name: Docker
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
+
+permissions:
+  packages: write
+
+jobs:
+  build:
+    name: Build image
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v6
+
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v4
+
+      - name: Docker metadata
+        id: meta
+        uses: docker/metadata-action@v6
+        with:
+          images: ghcr.io/${{ github.repository_owner }}/nantian-controlplane
+          tags: |
+            type=sha,prefix=sha-,format=short
+            type=raw,value=latest,enable=${{ github.ref == 'refs/heads/main' }}
+
+      - name: Login to GHCR
+        if: github.ref == 'refs/heads/main'
+        uses: docker/login-action@v4
+        with:
+          registry: ghcr.io
+          username: ${{ github.actor }}
+          password: ${{ secrets.GITHUB_TOKEN }}
+
+      - name: Build and push
+        uses: docker/build-push-action@v7
+        with:
+          context: .
+          push: ${{ github.ref == 'refs/heads/main' }}
+          tags: ${{ steps.meta.outputs.tags }}
+          labels: ${{ steps.meta.outputs.labels }}
+          cache-from: type=gha
+          cache-to: type=gha,mode=max