add support for RMENV in Dockerfile

RMENV allow the Dockerfile to remove the parent's ENVs. The syntax is:
> RMENV ENV1 ENV2 ...
ENV1 and ENV2 will be removed from the final image if they exist in the parent.

Docker-DCO-1.1-Signed-off-by: Daniel, Dao Quang Minh <dqminh89@gmail.com> (github: dqminh)