docker: do not run build-frontend on push

.drone.yml

@@ -42,6 +42,8 @@ steps:
       branch:
         - master
         - staging
+      event:
+        - pull_request
 
   - name: image-backend
     image: plugins/docker
@@ -80,8 +82,6 @@ steps:
         from_secret: REGISTRY_PASS
       dockerfile: web/Containerfile
       context: web
-    depends_on:
-      - build-frontend
     when:
       branch:
         - master
@@ -149,6 +149,6 @@ steps:
         - pull_request
 ---
 kind: signature
-hmac: c4222688ca348daadee16490168aafe9a3a3dd0c835954296ddb1bddddf0df4b
+hmac: cc7f2f8dac4285b5fa1df163bd92115f1a51a92050687cd08169e17803a2de4c
 
 ...