Alternative template using hosted /nix/store for use on user-controlled systems (e.g. VMs)

[bbarker]

There are numerous performance improvements that this could entail:

1. No need to initialize the nix repository upon insantiation of the container for the first times
2. Fewer checks each time the container is started (at least, I think so).
3. Packages could be shared between users on the VM, but this requires nix be installed with setuid accordingly - probably the standard nix installer handles this, but am unsure - certainly it is handled appropriately in NixOS
4. No need to use a beefy overlay image (may not need overlay at all).

Of course, there are some places where this won't work:

1. Where we don't control the nodes - e.g., Stampede or other large HPC systems. We can't mess with setuid there.
2. Where we really want to provide image snapshots for ultimate reproducibility (still, this comes in pairs: the base image and the overlay image).

[bbarker]

Bumping priority as apparently overlay images are not planned to be supported on Stampede due to security policy. Need to check if setuid is still an issue though.

We should probably still attempt to package up a tarball or something of the /nix/store/ to speed things up.