[containerization] Use image name rather than tag name.

docs/Using-Docker.md

 
 ```
 docker run --mount type=bind,source="$(pwd)"/unity-volume,target=/unity-volume \
-	 <tag-name>:latest <environment-name> \
+	 <image-name>:latest <environment-name> \
 	 --docker-target-name=unity-volume \
 	 --train --run-id=<run-id>
 ```