Create docker instance of pycsw using in-memory SQLite

[IanMayo]

We are switching strategy, to trial the pycsw backend indexing server.

Modify the README.md to explain the steps to pull then run a pycsw server. Specify the use of a config file that uses an in-memory SQLite database (we'll upgrade that in the future, but this is an easy first step).

Here are the docs for an in-memory database:

Note The easiest and fastest way to deploy pycsw is to use SQLite3 as the backend. To use an SQLite in-memory database, in the pycsw configuration, set repository.database to sqlite://.