Add GitHub Actions workflow for GHCR

This workflow builds and pushes Docker images to GitHub Container Registry (GHCR) for tagged releases and pull requests.

.github/workflows/ghcr.yml

@@ -0,0 +1,75 @@
+# build and push releases to ghcr
+# build for PRs only to test failures
+
+name: build-and-push-ghcr
+
+on:
+  push:
+    tags: ["v*"]
+  pull_request:
+
+permissions:
+  contents: read
+  packages: write
+
+concurrency:
+  group: ghcr-${{ github.ref }}
+  cancel-in-progress: true
+
+jobs:
+  docker:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Set up QEMU
+        uses: docker/setup-qemu-action@v3
+
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v3
+
+      - name: Log in to GHCR
+        if: github.event_name != 'pull_request'
+        uses: docker/login-action@v3
+        with:
+          registry: ghcr.io
+          username: ${{ github.actor }}
+          password: ${{ secrets.GITHUB_TOKEN }}
+
+      - name: Docker meta (tags/labels)
+        id: meta
+        uses: docker/metadata-action@v5
+        with:
+          images: ghcr.io/${{ github.repository_owner }}/job-ops
+          tags: |
+            type=ref,event=tag
+            type=sha
+            # Optional: also publish :latest for version tags
+            type=raw,value=latest
+
+      - name: Build (PR)
+        if: github.event_name == 'pull_request'
+        uses: docker/build-push-action@v6
+        with:
+          context: .
+          file: ./Dockerfile
+          push: false
+          platforms: linux/amd64,linux/arm64
+          tags: ${{ steps.meta.outputs.tags }}
+          labels: ${{ steps.meta.outputs.labels }}
+          cache-from: type=gha
+          cache-to: type=gha,mode=max
+
+      - name: Build and push (tag)
+        if: github.event_name != 'pull_request'
+        uses: docker/build-push-action@v6
+        with:
+          context: .
+          file: ./Dockerfile
+          push: true
+          platforms: linux/amd64,linux/arm64
+          tags: ${{ steps.meta.outputs.tags }}
+          labels: ${{ steps.meta.outputs.labels }}
+          cache-from: type=gha
+          cache-to: type=gha,mode=max