update CONTRIBUTING.md regarding build environments history

Author: OriHoch

CONTRIBUTING.md

@@ -94,4 +94,12 @@ Run the tests suite:
 
 ```
 tests/test_all.sh
-```
+```
+
+## Build Environments History
+
+Each build does fresh pulls of system dependencies and go modules.
+
+To revert or debug previous build environments, travis-ci pushed the build image to Docker Hub.
+
+The image tag is `cloudwm/cloudcli:{GIT_COMMIT_SHA}-{GOOS}-{GOARCH}`