Support multiple OSes besides Ubuntu 16.04

[ltirrell]

In reference to @pwighton's comments in #2 and #3:

Support for multiple operating systems is a great point, goes down to the scope of this repo.

The changes I made are more in line with supporting our use cases of having an Ubuntu 16 environment to build and run a dev version of FS. Because it may be beneficial to us (as well as other users), it'd be nice to also release packaged versions including prebuilt FS6/dev so building isn't necessary.

It definitely makes sense to support other OSes for testing purposes, as FreeSurfer officially uses CentOS, and eventually Ubuntu 16 users will want to move to Ubuntu 18. Whether/when we support these other versions probably depends on amount of time we put into this (creating an Ubuntu 18 container should be very simple, but CentOS has yum instead of apt-get so there will need to be at least minor changes), and my thought would be to add them as needed.

It would be nice if we could push multilevel naming to docker hub, like:

corticometrics/freesurfer-run/xenial:version_tag

But we're stuck with only one level of naming, and a tag. Your fs-dev-xenial-build naming convention gets around that, though then creates a new docker hub repository for each different OS we support. We could also do something like:

corticometrics/freesurfer-run:xenial

Let me know what you think!