non root user

[JoshCu]

When running the container on linux all created files are owned by the default user root. Adding --user 1000:1000 to the docker run command will execute it as that user so all created files will be owned by whichever user has uid 1000 (by default the first user added to the machine on ubuntu).
To make it easier to run the container as a non-root user, we can create some new user at the end of the build process and make sure that user has all the permissions needed to run ngen.
This is not as much of an issue on wsl and mac but would make things easier to use for linux executions.
I've looked into this a few times but I'm still not 100% sure this is the best way to go, open to suggestions. This isn't urgent but is inconvenient for me at least so I'll experiment some more and make a PR if nobody else gets a chance to look at this.