ci(gitlab-ci): add wait job for frontend container in merge request pipelines

.gitlab-ci.yml

@@ -79,13 +79,14 @@ setup:containers:frontend:
   needs:
     - setup:dynamic
   image: ${IMAGE_BUILDER}
-  script:
+  before_script: &container-before
     - FRONTEND_IMAGE_VERSION=`cat frontend-image-version`
     - IMAGE_TAG=frontend/${CI_COMMIT_REF_SLUG}:${FRONTEND_IMAGE_VERSION}
     - REGISTRY_DESTINATION=${CI_REGISTRY_IMAGE}/frontend/${CI_COMMIT_REF_SLUG}:${FRONTEND_IMAGE_VERSION}
+  script:
     - curl --request GET --header "PRIVATE-TOKEN:${REGISTRY_AUTH_TOKEN}" "${CI_API_V4_URL}/projects/${CI_PROJECT_ID}/registry/repositories?tags=true" > .gitlab-ci/.container-registry-contents.json
-    - cat .gitlab-ci/.container-registry-contents.json
     - IMAGE_EXISTS=`grep \""${REGISTRY_DESTINATION}"\" .gitlab-ci/.container-registry-contents.json`
+    - cat .gitlab-ci/.container-registry-contents.json
     - echo ${IMAGE_EXISTS}
     - echo ${CI_JOB_MANUAL}
     - test ${CI_JOB_MANUAL} == true && echo "Force rebuilding container."
@@ -98,7 +99,24 @@ setup:containers:frontend:
         echo "Image ${IMAGE_TAG} already exists in the container registry. Skipping build."
       fi
   rules:
+    - if: $CI_MERGE_REQUEST_ID
+      when: never
     - when: always
+setup:containers:frontend:wait:
+  stage: setup
+  needs:
+    - setup:dynamic
+  before_script: *container-before
+  script:
+    - >
+      while ! curl --request GET --header "PRIVATE-TOKEN:${REGISTRY_AUTH_TOKEN}" "${CI_API_V4_URL}/projects/${CI_PROJECT_ID}/registry/repositories?tags=true" | grep \""${REGISTRY_DESTINATION}"\" ; do
+        echo "Waiting for container ${IMAGE_TAG} to appear in the container registry..."
+        sleep 5
+      done
+  rules:
+    - if: $CI_MERGE_REQUEST_ID
+      when: always
+    - when: never
 
 # TODO: switch to exec logic as above (see frontend container)
 setup:containers:backend: &containers-backend