docs: remove a trailing whitespace

Signed-off-by: Brandon Philips <brandon.philips@coreos.com>
docs: use systemd drop-ins instead of copying

Copying the entire docker service file isn't necessary to add an
environment variable, instead use a drop-in configuration file. The nice
side-effect is that the user gets any vendor updates to the
docker.service file.

Signed-off-by: Brandon Philips <brandon.philips@coreos.com>
Merge pull request #9559 from philips/use-dropins-in-systemd-article

docs: use systemd drop-ins instead of copying