Unexpected behavior of docker-gc

[dalanlan]

1 Run as a binary
Initialization: no /etc/docker-gc-exclude && /etc/docker-gc-exclude-containers files ( empty EXCLUDE_FROM_GC && EXCLUDE_CONTAINERS_FROM_GC ENV)
Run: After the execution, i would get a /var/lib/docker-gc/exclude_container_ids file containing all of the existing containers, while /var/lib/docker-gc/exclude_ids is empty, which is not expected.

2 Run as a docker container
Simple docker run --rm -v /var/run/docker.sock:/var/run/docker.sock spotify/docker-gc is not gonna work.
How could it possibly function well without accessing EXCLUDE_FROM_GC ENV (sort of stuff)? 'Cause it's located in /etc/docker-gc-exclude-containers.
We need a volume at least.

[davidxia]

@dalanlan I think this PR fixes 1 right? #46

[dalanlan]

Hmm.. good to go then.