ci: auto-publish dashboard docker image on v* tag (closes #122)

Adds a docker-publish job to publish.yml that runs after the NuGet
publish succeeds on a v<version> tag push. Builds from the existing
docker/dashboard/Dockerfile and pushes:

  blehnen74/dotnetworkqueue-dashboard:<version>
  blehnen74/dotnetworkqueue-dashboard:latest

Dry-run semantics match the NuGet publish job -- the docker job is
skipped entirely when workflow_dispatch is invoked with dry_run=true,
so operational dry-runs exercise verify-gate + build-pack without
either NuGet or docker side effects.

Single-platform (linux/amd64) for now; issue #122 lists multi-arch
as an optional follow-up. DOCKERHUB_USERNAME and DOCKERHUB_TOKEN
secrets must be configured in repo settings before first run.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: blehnen

.github/workflows/publish.yml

@@ -196,3 +196,48 @@ jobs:
             echo "- NuGet feed: https://api.nuget.org/v3/index.json"
             echo "- Release URL: https://github.com/${{ github.repository }}/releases/tag/${{ github.ref_name }}"
           } >> "$GITHUB_STEP_SUMMARY"
+
+  docker-publish:
+    name: Build and push dashboard Docker image
+    needs: publish
+    # Same dry-run semantics as the NuGet publish job: skipped entirely when
+    # workflow_dispatch is invoked with dry_run=true.
+    if: ${{ github.event_name == 'push' || inputs.dry_run == false }}
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Resolve version from tag
+        id: ver
+        run: echo "version=${GITHUB_REF_NAME#v}" >> "$GITHUB_OUTPUT"
+
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v3
+
+      - name: Log in to Docker Hub
+        uses: docker/login-action@v3
+        with:
+          username: ${{ secrets.DOCKERHUB_USERNAME }}
+          password: ${{ secrets.DOCKERHUB_TOKEN }}
+
+      - name: Build and push image
+        uses: docker/build-push-action@v6
+        with:
+          context: .
+          file: docker/dashboard/Dockerfile
+          platforms: linux/amd64
+          push: true
+          tags: |
+            blehnen74/dotnetworkqueue-dashboard:${{ steps.ver.outputs.version }}
+            blehnen74/dotnetworkqueue-dashboard:latest
+
+      - name: Job summary
+        run: |
+          {
+            echo "## Docker publish summary"
+            echo ""
+            echo "- Image: blehnen74/dotnetworkqueue-dashboard"
+            echo "- Tags pushed: ${{ steps.ver.outputs.version }}, latest"
+            echo "- Platforms: linux/amd64"
+          } >> "$GITHUB_STEP_SUMMARY"