Add documentation to docker_test.rb

script/docker_test.rb

@@ -1,3 +1,12 @@
+# This script is run in the discourse_test docker image
+# Available environment variables:
+# => 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
+#
+# See lib/tasks/docker.rake for other available environment variables
+
 def run_or_fail(command)
   pid = Process.spawn(command)
   Process.wait(pid)