fix: CI Docker tags must be lowercase, upgrade docker actions

- Use hardcoded lowercase image prefix instead of github.repository
- Upgrade docker/login-action to v4 and docker/build-push-action to v7

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: Romain-Grosos

.github/workflows/ci.yml

@@ -6,6 +6,10 @@ on:
   pull_request:
     branches: [main]
 
+env:
+  REGISTRY: ghcr.io
+  IMAGE_PREFIX: ghcr.io/rwx-g/stackoverkill.io
+
 jobs:
   validate:
     name: Lint, Typecheck & Test
@@ -53,28 +57,28 @@ jobs:
       - uses: actions/checkout@v5
 
       - name: Log in to GitHub Container Registry
-        uses: docker/login-action@v3
+        uses: docker/login-action@v4
         with:
-          registry: ghcr.io
+          registry: ${{ env.REGISTRY }}
           username: ${{ github.actor }}
           password: ${{ secrets.GITHUB_TOKEN }}
 
       - name: Build & push frontend image
-        uses: docker/build-push-action@v6
+        uses: docker/build-push-action@v7
         with:
           context: .
           file: docker/frontend.Dockerfile
           push: true
           tags: |
-            ghcr.io/${{ github.repository }}/frontend:latest
-            ghcr.io/${{ github.repository }}/frontend:${{ github.sha }}
+            ${{ env.IMAGE_PREFIX }}/frontend:latest
+            ${{ env.IMAGE_PREFIX }}/frontend:${{ github.sha }}
 
       - name: Build & push backend image
-        uses: docker/build-push-action@v6
+        uses: docker/build-push-action@v7
         with:
           context: .
           file: docker/backend.Dockerfile
           push: true
           tags: |
-            ghcr.io/${{ github.repository }}/backend:latest
-            ghcr.io/${{ github.repository }}/backend:${{ github.sha }}
+            ${{ env.IMAGE_PREFIX }}/backend:latest
+            ${{ env.IMAGE_PREFIX }}/backend:${{ github.sha }}