Instructions for docker user namespaces

[Jack Hale]

Linux users see issues as uids in the container are replicated exactly outside of the container. So, fenics in the container is UID 1000, and the user on the host is UID 501, you get crazy file permissions that just don't work anywhere.

Docker now has new user namespaces, which allow mapping of UIDs/GIDs in the container onto UIDs/GIDs outside the container. This functionality should be implemented into:

1) The FEniCS Docker documentation for advanced users. 2) The fenicsproject script.

[Jack Hale]

Suggested a much easier path to getting file permissions right in pull request #11.

[Jack Hale]

Don't see a pressing need for this.