Use nix based docker base image

[sveitser]

Motivation

1. No need for patchelf. Base image inputs are tracked with nix.
2. Make it possible to build the image locally. Should make it easier to put together the remaining images we need.

[sveitser]

Building the base image works on the CI but the final "Build and Push Docker" step fails. I think it's because it's trying to fetch metadata about the base image from dockerhub and that image doesn't exist.

https://github.com/EspressoSystems/cape/runs/5511464159?check_suite_focus=true

[sveitser]

Possibly relevant: docker/build-push-action#425 (comment)

[sveitser]

It does seem to build successfully with the docker driver.

[Ancient123]

Discussed with @sveitser via chat. Approved.