support for `docker run` environment variables file

Docker-DCO-1.1-Signed-off-by: Vincent Batts <vbatts@redhat.com> (github: vbatts)
pkg/opts: Close the file handle

Docker-DCO-1.1-Signed-off-by: Vincent Batts <vbatts@redhat.com> (github: vbatts)
--env-file instead of --envfile

Docker-DCO-1.1-Signed-off-by: Vincent Batts <vbatts@redhat.com> (github: vbatts)
go fmt

Docker-DCO-1.1-Signed-off-by: Vincent Batts <vbatts@redhat.com> (github: vbatts)
Fixing doc references to --env-file

Docker-DCO-1.1-Signed-off-by: Vincent Batts <vbatts@redhat.com> (github: vbatts)
make the --env-file accept multiple flags

Docker-DCO-1.1-Signed-off-by: Vincent Batts <vbatts@redhat.com> (github: vbatts)
env-file: update functionality and docs

Multiple flags allowed. Order prescribed. Examples provided. Multiline
accounted for.

Docker-DCO-1.1-Signed-off-by: Vincent Batts <vbatts@redhat.com> (github: vbatts)
env-file: remove the unneeded deprecation markup

Docker-DCO-1.1-Signed-off-by: Vincent Batts <vbatts@redhat.com> (github: vbatts)
--env-file: simple line-delimited

match dock functionality, and not try to achieve shell-sourcing compatibility

Docker-DCO-1.1-Signed-off-by: Vincent Batts <vbatts@redhat.com> (github: vbatts)
env-file: variable behavior

trim the front of variables. Error if there are other spaces present.
Leave the value alone.

Docker-DCO-1.1-Signed-off-by: Vincent Batts <vbatts@redhat.com> (github: vbatts)
Merge pull request #4174 from vbatts/vbatts-env_file

support for `docker run` environment variables file
integration-cli: pull busybox before running

Make sure the busybox image is ready to be used when running the cli
integration tests.

Docker-DCO-1.1-Signed-off-by: Cristian Staretu <cristian.staretu@gmail.com> (github: unclejack)