ci: publish Docker image to GHCR on push to main

Builds and pushes ghcr.io/r0m1k3/xtremflow:latest (+ sha tag) so update
managers (Unraid/Watchtower/Portainer) can detect new images instead of
relying on a local build context.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

.github/workflows/docker-publish.yml

@@ -0,0 +1,39 @@
+name: Docker Publish
+
+on:
+  push:
+    branches: [main]
+  workflow_dispatch:
+
+env:
+  IMAGE_NAME: ghcr.io/r0m1k3/xtremflow
+
+jobs:
+  build-and-push:
+    runs-on: ubuntu-latest
+    permissions:
+      contents: read
+      packages: write
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: docker/setup-buildx-action@v3
+
+      - name: Log in to GHCR
+        uses: docker/login-action@v3
+        with:
+          registry: ghcr.io
+          username: ${{ github.actor }}
+          password: ${{ secrets.GITHUB_TOKEN }}
+
+      - name: Build and push
+        uses: docker/build-push-action@v6
+        with:
+          context: .
+          file: Dockerfile
+          push: true
+          tags: |
+            ${{ env.IMAGE_NAME }}:latest
+            ${{ env.IMAGE_NAME }}:${{ github.sha }}
+          cache-from: type=gha
+          cache-to: type=gha,mode=max