Docker run -e FOO should erase FOO if FOO isn't set in client env
Docker run -e FOO should erase FOO if FOO isn't set in client env

See #10141 for more info, but the main point of this is to make sure
that if you do "docker run -e FOO ..." that FOO from the current env
is passed into the container.  This means that if there's a value, its
set.  But it also means that if FOO isn't set then it should be unset in
the container too - even if it has to remove it from the env.  So,
   unset HOSTNAME
   docker run -e HOSTNAME busybox env
should _NOT_ show HOSTNAME in the list at all

Closes #10141

Signed-off-by: Doug Davis <ff3bde741799c0eff815b83a54eda3fc494df54f@us.ibm.com>

e11137/cli,Zebrilee/cli,riyazdf/cli,ashfall/cli,allencloud/cli,thaJeztah/cli,Zebrilee/cli,cpuguy83/cli,redpanda/cli,e11137/cli,ashfall/cli,redpanda/cli,riyazdf/cli,thaJeztah/cli,dnephin/cli,docker/cli,cpuguy83/cli,docker/cli,allencloud/cli,dnephin/cli