Prevent removal of Data Containers

[MrMMorris]

Just noticed my hourly docker-gc cron was removing my data only containers 😱

Is the only way to prevent this to add them as an exception? Could docker-gc check for VolumesFrom?

[anroots]

I failed to RTFM and so met this exact case. mariadb container was not removed as it was running, but bye-bye mariadb-data, which was a stopped, linked data-only container.

Adding a "exclude containers" feature similar to image exclusion would resolve this.

[rohansingh]

Fixed by #38.