Allow a few more DOCKER_* env vars to pass thru Makefile

I was trying to just build the Docker client but DOCKER_CLIENTONLY wasn't
getting passed thru from the shell to the container building docker.
I checked for other DOCKER_* env vars in hack/make.sh to see what other
ones might be candidates to be passed along and just DOCKER_GITCOMMIT
jumped out at me.

So, this PR passes those vars (via the -e option) on the docker run command
so we pick them up from the devs shell when running "make ...".

While in there I pulled all of the "-e" options into a new Makefile variable
so its easy to see just the list of env vars we pass along.

Signed-off-by: Doug Davis <dug@us.ibm.com>