build: add shell.nix and minimal flake.nix

[lionello]

with build dependencies.

• Tested with --pure;
• Fixes running just build and just docker on clean MacOS with Nix (and Docker desktop.)
• Updated the readme (also mentioning fetch, without which just build fails.)
• Added a Github action

[lionello]

Closing since I want to add CI before I open the PR.

[lionello]

@olix0r I understand completely, and yes, devcontainers take care of the dev experience parts. I opened the PR mostly so other Nix users would have a starting point. It's OK with me if we close this PR and leave it at that. People can still find it if they search for "nix".