Remove local port bindings for db

@DSASanFrancisco/portal-members

This PR removes host port bindings from the docker-compose.yml. This causes issues when attempting to deploy two instances of the portal (e.g. for separate environments) on the same machine, and isn't necessary for normal operation.

I am making this change on docker-compose.yml because it is impossible to remove the port binding with an overriding yml file i.e. you can't make a file docker-compose.unmap-ports.yml and run docker-compose -f docker-compose.yml -f docker-compose.unmap-ports.yml up to fix this issue.

This PR amends the README to reflect changes in using external tools with the DB container.