Add documentation for new ENV variables

lib/tasks/docker.rake

@@ -14,6 +14,8 @@
 # Other useful environment variables (not specific to this rake task)
 # => COMMIT_HASH    used by the discourse_test docker image to load a specific commit of discourse
 #                   this can also be set to a branch, e.g. "origin/tests-passed"
+# => SKIP_LINT      if using the discourse_test docker image, this will skip the docker:test task, and only run the docker:lint task
+# => LINT_ONLY      if using the discourse_test docker image, this will skip the docker:lint task, and only run the docker:test task
 #
 # Example usage:
 #   Run all core and plugin tests: