Add pgschema to nix package

[JannisKoeksel]

As the title says, it would be super helpful to have it as a nix package.
For our case it would be used twofold:

1. Being used in the local env during development without having to install for each new mashine
2. We are building our container with nix and created a little K8s operator around pgschema, which is the only tool that needs the Docker image.

[tianzhou]

I have little experience with nix. Contribution welcome

[JannisKoeksel]

Not familiar with Go, but the Dockerfile looks simple enough. I will look into it.