Add nix flake with devshell and package

[robbevp]

re accentor/flake#5

This PR:

• Adds a flake.nix that exposes the api package and a devshell
• Adds a github action to automatically keep gemset.nix in sync (after dependabot or if people don't use nix).

Might be good if this can be tried on different machines and OS's (I only have aarch64-darwin).

Details

The devshell can be used with nix develop or use flake inside an .envrc-file.

It will print out a message with some of the additional commands:

🔨 Welcome to Accentor API

[database]

  pg:console - Open database console
  pg:setup   - Setup postgres in project folder
  pg:start   - Start postgres instance
  pg:stop    - Stop postgres instance

[general commands]

  menu       - prints this menu

If we want to, we can customize this message.

The pg:* commands allow us to keep the database fully contained within the project folder. Running pg:start should be plenty to setup and run a database in tmp/postgres. (I adopted this approach so I can easily switch between major versions of postgres accross different projects).

Github action

@chvp For this action to work after dependabot (and trigger actions), you might have to add a PAT for dependabot to use. source

[chvp]

Played around with it locally, everything seems to work.