Update build.yml

Author: Lannro

.github/workflows/build.yml

@@ -43,7 +43,7 @@ jobs:
       env:
         REGISTRY: ${{ steps.login-ecr.outputs.registry }}
         REPOSITORY: jhub-open-userimage
-        IMAGE_TAG: ${{ github.ref }}
+        IMAGE_TAG: ${{ github.sha }}
       run: |
         docker build -t $REGISTRY/$REPOSITORY:$IMAGE_TAG .
         docker push $REGISTRY/$REPOSITORY:$IMAGE_TAG