Using official postgres image

Using the [official postgres image](https://registry.hub.docker.com/_/postgres/) (instead of [orchardup/postgresql](https://registry.hub.docker.com/u/orchardup/postgresql/)) to match the [whitesmith/fig-tree/rails-pg-fig-devenv](https://github.com/whitesmith/fig-tree/tree/1e6f3bfc5b106cf76977d54cb9e957f09dddd12b/rails-pg-fig-devenv) setup; updating the `config/database.yml` accordingly.

Removing the Ruby version pinning and updating the Gemfile.

Some additional comments added to `README.md`.