Use separate cache scopes for frontend and worker

Otherwise, the cache is effectively discarded since the same images
aren't used for both builds and the cache therefore assumes they're
no longer in use and garbage-collects them.

See docker/build-push-action#252 (comment)

.github/workflows/ci.yaml

@@ -97,8 +97,8 @@ jobs:
           context: .
           push: true
           tags: lsstsqre/vo-cutouts:${{ steps.vars.outputs.tag }}
-          cache-from: type=gha
-          cache-to: type=gha,mode=max
+          cache-from: type=gha,scope=frontend
+          cache-to: type=gha,mode=max,scope=frontend
 
       - name: Build and push the worker image
         uses: docker/build-push-action@v2
@@ -107,5 +107,5 @@ jobs:
           push: true
           file: ./Dockerfile.worker
           tags: lsstsqre/vo-cutouts-worker:${{ steps.vars.outputs.tag }}
-          cache-from: type=gha
-          cache-to: type=gha,mode=max
+          cache-from: type=gha,scope=worker
+          cache-to: type=gha,mode=max,scope=worker