Add BUILD_ASSETS_IMAGE CI variable

* Defaults to FALSE
* Can be used to disable building/pushing the assets image in the
gitlab:compile:assets job

.gitlab-ci.yml

@@ -32,7 +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: "true"
+  BUILD_ASSETS_IMAGE: "false"
   ## EE specific variables ##
   # This hack is needed to make ES not that memory hungry
   ES_JAVA_OPTS: "-Xms256m -Xmx256m"