ci: add GitHub Actions auto-build + fix Unraid update

- Add .github/workflows/docker-publish.yml: builds Docker image on every
  push to main and publishes to ghcr.io/r0m1k3/podcastic:latest automatically

- docker-compose.yml: replace local `build:` with `image: ghcr.io/r0m1k3/podcastic:latest`
  so Unraid's "Update" button can pull the image directly from the registry
  instead of failing with "no service selected"

- Remove deprecated `version: '3.8'` field

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.github/workflows/docker-publish.yml

@@ -0,0 +1,49 @@
+name: Build & Publish Docker Image
+
+on:
+  push:
+    branches: [main]
+
+env:
+  REGISTRY: ghcr.io
+  IMAGE_NAME: ${{ github.repository }}
+
+jobs:
+  build-and-push:
+    runs-on: ubuntu-latest
+    permissions:
+      contents: read
+      packages: write
+
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v4
+
+      - name: Log in to GitHub Container Registry
+        uses: docker/login-action@v3
+        with:
+          registry: ${{ env.REGISTRY }}
+          username: ${{ github.actor }}
+          password: ${{ secrets.GITHUB_TOKEN }}
+
+      - name: Extract metadata (tags, labels)
+        id: meta
+        uses: docker/metadata-action@v5
+        with:
+          images: ${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}
+          tags: |
+            type=raw,value=latest
+            type=sha,prefix=sha-
+
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v3
+
+      - name: Build and push Docker image
+        uses: docker/build-push-action@v5
+        with:
+          context: .
+          push: true
+          tags: ${{ steps.meta.outputs.tags }}
+          labels: ${{ steps.meta.outputs.labels }}
+          cache-from: type=gha
+          cache-to: type=gha,mode=max

docker-compose.yml

@@ -1,5 +1,3 @@
-version: '3.8'
-
 services:
   mongodb:
     image: mongo:8
@@ -31,9 +29,7 @@ services:
       retries: 5
 
   app:
-    build:
+    image: ghcr.io/r0m1k3/podcastic:latest
-      context: .
-      dockerfile: Dockerfile
     container_name: podcastic-app
     restart: unless-stopped
     ports: