Add GitHub Actions workflow for Docker builds

Builds and pushes images to GitHub Container Registry:
- ghcr.io/clucraft/priceghost-backend
- ghcr.io/clucraft/priceghost-frontend

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

.github/workflows/docker-build.yml

@@ -0,0 +1,96 @@
+name: Build and Push Docker Images
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+    branches: [main]
+  workflow_dispatch:
+
+env:
+  REGISTRY: ghcr.io
+  BACKEND_IMAGE: ghcr.io/${{ github.repository_owner }}/priceghost-backend
+  FRONTEND_IMAGE: ghcr.io/${{ github.repository_owner }}/priceghost-frontend
+
+jobs:
+  build-backend:
+    runs-on: ubuntu-latest
+    permissions:
+      contents: read
+      packages: write
+
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v4
+
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v3
+
+      - name: Log in to GitHub Container Registry
+        if: github.event_name != 'pull_request'
+        uses: docker/login-action@v3
+        with:
+          registry: ${{ env.REGISTRY }}
+          username: ${{ github.actor }}
+          password: ${{ secrets.GITHUB_TOKEN }}
+
+      - name: Extract metadata for Backend
+        id: meta
+        uses: docker/metadata-action@v5
+        with:
+          images: ${{ env.BACKEND_IMAGE }}
+          tags: |
+            type=ref,event=branch
+            type=sha,prefix=
+            type=raw,value=latest,enable={{is_default_branch}}
+
+      - name: Build and push Backend image
+        uses: docker/build-push-action@v5
+        with:
+          context: ./backend
+          push: ${{ github.event_name != 'pull_request' }}
+          tags: ${{ steps.meta.outputs.tags }}
+          labels: ${{ steps.meta.outputs.labels }}
+          cache-from: type=gha
+          cache-to: type=gha,mode=max
+
+  build-frontend:
+    runs-on: ubuntu-latest
+    permissions:
+      contents: read
+      packages: write
+
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v4
+
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v3
+
+      - name: Log in to GitHub Container Registry
+        if: github.event_name != 'pull_request'
+        uses: docker/login-action@v3
+        with:
+          registry: ${{ env.REGISTRY }}
+          username: ${{ github.actor }}
+          password: ${{ secrets.GITHUB_TOKEN }}
+
+      - name: Extract metadata for Frontend
+        id: meta
+        uses: docker/metadata-action@v5
+        with:
+          images: ${{ env.FRONTEND_IMAGE }}
+          tags: |
+            type=ref,event=branch
+            type=sha,prefix=
+            type=raw,value=latest,enable={{is_default_branch}}
+
+      - name: Build and push Frontend image
+        uses: docker/build-push-action@v5
+        with:
+          context: ./frontend
+          push: ${{ github.event_name != 'pull_request' }}
+          tags: ${{ steps.meta.outputs.tags }}
+          labels: ${{ steps.meta.outputs.labels }}
+          cache-from: type=gha
+          cache-to: type=gha,mode=max