make docker run directly under systemd without forking

Docker seems to be working fine now under systemd. Some recent code
changes have fixed the problems we were running into before.

What has been tested while running docker directly under systemd:
1. docker run
2. docker pull
3. docker images
4. docker rm
Merge pull request #38 from unclejack/run_docker_service_without_forking

make docker run directly under systemd without forking