Postgres Docker image

Initially in the CoreDB operator, we were using Docker's official Postgres image https://github.com/docker-library/postgres with no modifications.

Now, we are looking into turning off or on extensions using the CoreDB operator. First, we want to address turning off or on extensions from a list of supported extensions that are already installed. Then, we want to address arbitrary, user-provided extensions.

For initial testing, we are installing some arbitrary extensions from this APT repository https://wiki.postgresql.org/wiki/Apt

Versioning

The version of the Docker image can be configured in the Cargo.toml file in this directory. We may wrap postgres in our CoreDB distribution, but for the time being this crate is just a placeholder to allow for versioning.