Update readme

Author: GilbN

Diff for: readme-vars.yml

@@ -55,6 +55,7 @@ full_custom_readme: |
   -e DOCKER_LOGS_TIMEOUT=<optional, How long to wait in seconds while tailing the container logs before timing out. Defaults to '120'> \
   -e DRY_RUN=<optional, Set to 'true' when you don't want to upload files to S3 when testing> \
   -e NODE_NAME=<optional, Name of the builder that runs the CI test.> \
+  -e RELEASE_TAG=<optional, The release tag of the docker image. Used for upload location. Defaults to 'latest'> \
   -t lsiodev/ci:latest \
   python3 test_build.py
   ```