1. Ignore unnecessary files, such as `pgcli` log/history, and extra `docker-compose` definitions.
2. Add `Makefile` with common tasks.
3. Add git pre-commit hook to ensure project is always properly formatted.
4. Add [`btree_gist`][0] extension to `postgres`.
Reviewed-on: #6
[0]: https://www.postgresql.org/docs/14/btree-gist.html
* Upgrade elixir, erlang, and postgres to their latest versions.
* Fix CI configuration, so tests are executed.
* Migrate project configuration from `Mix.Config` to `Config`.
Reviewed-on: #5
