No more exporting pause by running the app process in buildkit

[richiejp]

When you build large containers (1GB+) in Buildkit there is this pause where it exports to image. Even if most stuff is cached I still get a pause of approx 2 mins on a CUDA image. Really annoying when changing one line of Python. For development I want a really tight feedback loop.

So I thought why not just run the application in Buildkit? Which is what I have done and now there is no pause.

This also means that Ayup no longer requires containerd or Nerdctl and can be run as a normal user with Rootlesskit. So soon as I figure out how to bundle Buildkit, Rootlesskit and runc in Ayup, installation will just a be case of downloading and running the exe.

On the downside Buildkit has little support for advanced network setups and port forwarding. Rootlesskit could make it worse or better depending how you look at it.

I'm not sure that the Buildkit devs wanted people to do this, but the client API actually has some nice support for running processes that aren't necessarily part of the build, so I'm going to run with it.

Opinions and questions welcome: #26