DevExp: local environment

This is a tracking issue for current problems when trying to bring up Flagsmith locally:

cc @gnumoreno

### Common

- [ ] No single documented way of doing this. We should also document prerequisites i.e. pyenv, Docker and npm.

### API

- [ ] `make install` failing due to Poetry not updating `$PATH` out of the box.
- [ ] `make serve` target should invoke `docker-up` and `django-migrate` targets automatically.

### Frontend

- [ ] `make build` target is incorrect, should invoke `npm run bundle` instead of `npm run build`.
- [ ] Should add `ENV=local` to `.env-local` to point frontend brought up by the `make serve` target to the local API.