Update Docker base image to version v3.3.0
Update Docker base image to version v3.3.0

This commit updates the base image of this app to the latest version of
v3.3.0 of the Digital Marketplace docker image.

v3.3.0 is a rolling release that could theoretically be updated at any
time, however initially this app has been tied to a specific build.

Ticket: https://trello.com/c/ldIdPBzy

alphagov/digitalmarketplace-api,alphagov/digitalmarketplace-api,alphagov/digitalmarketplace-api