Add a backup script to the postgres image

[martyngigg]

As a developer I want to easily take backups of the database by running a backup command in the database.

This cookie-cutter project does something like this. The key parts that we can base our solution on is using a custom postgres image that simply adds a basic backup script that calls pg_dump.

[martyngigg]

@Mantid-Matthew @DanielMurphy22 I think it would be great if we could look at this in maintenance. I've created mantidproject/reports#27 to do something similar for the usage reports. If we could workout sharing code that would be even better as there's so much common between these two projects.

[DavidFair]

Done in #21