support for `docker run` environment variables file

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

docs/sources/reference/commandline/cli.rst

@@ -1152,6 +1152,7 @@ image is removed.
       --cidfile="": Write the container ID to the file
       -d, --detach=false: Detached mode: Run container in the background, print new container id
       -e, --env=[]: Set environment variables
+      --envfile="": Read in a line delimited file of ENV variables
       -h, --hostname="": Container host name
       -i, --interactive=false: Keep stdin open even if not attached
       --privileged=false: Give extended privileges to this container
@@ -1284,6 +1285,17 @@ This exposes port ``80`` of the container for use within a link without
 publishing the port to the host system's interfaces. :ref:`port_redirection`
 explains in detail how to manipulate ports in Docker.
 
+.. code-block:: bash
+
+    $ sudo docker run -e MYVAR1 --env MYVAR2=foo --envfile ./env.list ubuntu bash
+
+This sets environmental variables to the container. For illustration all three
+flags are shown here. Where -e and --env can be repeated, take an environment 
+variable and value, or if no "=" is provided, then that variable's current
+value is passed through (i.e. $MYVAR1 from the host is set to $MYVAR1 in the
+container). The --envfile flag takes a filename as an argument and expects each
+line to be a VAR=VAL format.
+
 .. code-block:: bash
 
     $ sudo docker run --name console -t -i ubuntu bash