support for `docker run` environment variables file

With many environment variables individually added with -e/--env can/will overflow the shell argument max length $(getconf ARG_MAX). Having a file to declare environment variables will reduce this, as well as allow for automation of known/static environment variables.

```
$ cat e.list
FOO=BAR
BAZ=BIFF
$ ./bundles/0.8.0-dev/dynbinary/docker-0.8.0-dev run -i -t -envfile ./e.list -envfile ./e.list busybox sh
/ # echo $FOO $BAZ
BAR BIF
```

Docker-DCO-1.1-Signed-off-by: Vincent Batts <vbatts@redhat.com> (github: vbatts)