Support MergerFS in docker container

[ChrisWeiss]

There are two closed issues that revolve around permissions issues when running metube in a container pointing to a MergerFS volume (add myself to the list of blocked users):
#174
#121

It would be helpful to either solve the permissions issue, or (at the very least, instructions on how to) include MergerFS components in the Dockerfile.

[PikuZheng]

Alpine Linux support mergerfs on edge branch only. so maybe you want build your image with this
(not tested)