Merge branch 'conditionally-build-assets-image' into 'master'

Add BUILD_ASSETS_IMAGE CI variable

See merge request gitlab-org/gitlab-ce!23076

.gitlab-ci.yml

@@ -32,6 +32,7 @@ variables:
   GET_SOURCES_ATTEMPTS: "3"
   KNAPSACK_RSPEC_SUITE_REPORT_PATH: knapsack/${CI_PROJECT_NAME}/rspec_report-master.json
   FLAKY_RSPEC_SUITE_REPORT_PATH: rspec_flaky/report-suite.json
+  BUILD_ASSETS_IMAGE: "false"
 
 before_script:
   - bundle --version

scripts/build_assets_image

@@ -1,5 +1,11 @@
 #!/bin/bash
 
+# Exit early if we don't want to build the image
+if [[ "${BUILD_ASSETS_IMAGE}" != "true" ]]
+then
+    exit 0
+fi
+
 # Generate the image name based on the project this is being run in
 ASSETS_IMAGE_NAME=$(echo ${CI_PROJECT_NAME} |
    awk '{