Enable PostGIS on Database Initialization

When the container is created for the first time from within a docker-compose definition, a database is created with the given host, username, and password. At this point, we should also enable the PostGIS extension.

One option may be to use a PostGIS enabled database template, like the one mentioned here: https://docs.bitnami.com/aws/infrastructure/postgresql/configuration/create-postgis-template/

Currently, this enabling is done by sister containers (Django / Scala). However, since this image is named "postgis", it should not rely on an external step to enable that core functionality.

For further discussion, see here: https://azavea.slack.com/archives/C02DNK1QC/p1624293605188600

Start with the Docker image initialization path used when a container is first created from docker-compose, then review the linked Bitnami PostGIS template guidance. The work is complete when a fresh container creates the configured database with the PostGIS extension enabled without relying on a Django or Scala sister container.

Written by the indexing model from the issue text.