Postgres Docker image with contribution package enabled

This is used by the App Hub[1] to get access to the uuid-ossp[2] extension in Postgres.

When this was created the official Docker image for Postgres didn't include contrib modules, and the available ones on Docker Hub were too old for us to use because they weren't compatible with the Postgres version we are using on AWS RDS.

This can be deprecated if/when we:

• migrate to generating UUIDs on the Server in code instead of in the DB,
• use an official image that is kept up-to-date and has the contrib modules enabled.

1. https://github.com/dhis2/app-hub/blob/master/server/migrations/20191217103142_auto-uuid.js#L23
2. https://www.postgresql.org/docs/10/uuid-ossp.html