ci: fix backend image build (#418)

Signed-off-by: Alexandre-P <alexandre.picosson@owkin.com>

.github/workflows/gcr.yml → .github/workflows/container-image-backend.yml

@@ -12,6 +12,7 @@ on:
     paths:
       - 'docker/substra-backend/*'
       - 'backend/**'
+      - '.github/workflows/container-image-backend.yml'
 
 jobs:
   build:
@@ -31,18 +32,20 @@ jobs:
           echo "BACKEND_VERSION=`echo $GITHUB_REF | cut -d / -f 3`+`echo ${GITHUB_SHA} | cut -c1-8`" >> $GITHUB_ENV
         fi
 
-    - name: Login to GCR
+    - name: Login to registry
       uses: docker/login-action@v2
+      if: ${{ github.event_name != 'pull_request' }}
       with:
-        registry: gcr.io
-        username: _json_key
-        password: ${{ secrets.GCP_SA_KEY }}
+        registry: ${{ env.REGISTRY }}
+        username: ${{ github.actor }}
+        password: ${{ secrets.GITHUB_TOKEN }}
+
 
     - name: Docker metadata
       id: docker-metadata
       uses: docker/metadata-action@v4
       with:
-        images: 'gcr.io/${{ secrets.GCP_PROJECT_ID }}/connect-backend'
+        images: 'ghcr.io/substra/substra-backend'
         flavor: |
           latest=${{ env.LATEST }}