dockerize dev setup

[jdx]

Contributing to rtx is a bit painful. The E2E and unit tests can be impacted by the local environment. If you have rtx activate set, it will almost certainly do the wrong thing.

Rather than continue to try sandboxing everything, I feel like it would be better to do our development inside of a docker container. This will make it easier for contributors since it will have dependencies like direnv and fish already setup.

Ironically I haven't been able to use rtx myself yet for its intended purpose. I'm always resetting my environment and testing edge cases so it's basically always in a bad state. If I had this I might be able to actually use it myself.