Change Docker-Compose to use mmapv1 storage engine for mongo

in order to make preview work out of the box for everyone who is not using a proxy

And start using depends_on

Added options for MAIL and PROXY

Version 1.x.x was being used and it's not working right.

* Removing --replSet rs0 which causes issue as described here: https://github.com/RocketChat/Rocket.Chat/issues/1472
* Updating the mongo database datadir location so that if somebody decides to use volumes in order to maintain state, it works. I'm guessing mongodb changed it's default location from "mongo" to "db" and nobody noticed.

add support for mobile client for pre-built server binaries tgz

moved to Dockerfile

correct typos (in commented lines)

even though default is GridFS storing to mongo; for some reasons, many docker users still insist on changing it to FileSystem

use official repository image instead of my test repository