Postgres Docker image

Contains a Dockerfile with a set of standard set of community Postgres extensions, and utilities for managing Postgres with the coredb-operator. Over time, this image will be pruned down to the bare minimum required for the coredb-operator.

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.