Allow setting of environment variables when importing docker images

We want to be able to set the container=docker environment
variable when we build RHEL docker images.  I believe there
will be other use cases where you might want to set
environment in base images.

Since you can do this in layered image, I see no reason not
to allow it in base images.

Docker-DCO-1.1-Signed-off-by: Dan Walsh <dwalsh@redhat.com> (github: rhatdan)
Maybe it's the passive voice, but I had a hard time understanding this sentence. It's hard to tell if the variables are used by the process or the container. Can you revise it to make that clearer?