Dockerise this app

[benilovj]

Context

Some DfE teams are using Docker to deploy and run their Rails apps to production. It'd be helpful to dockerise this repo so that other teams can adopt this approach easily.

Background

There is prior art in GitHub already, see the Manage Courses Frontend:

DFE-Digital/publish-teacher-training-2022#46 (Switching to Alpine)
DFE-Digital/publish-teacher-training-2022#30 (DRYing the scripts)
DFE-Digital/publish-teacher-training-2022#11 (Original work)

This includes a yarn-backed asset pipeline.

Previous attempts

#26

[dankmitchell]

The setup on manage-courses-backend doesn't include Yarn and the setup on manage-courses-frontend doesn't include PostgreSQL.

Once https://github.com/DFE-Digital/manage-courses-support/pull/82/files is finished then we'll have a complete setup that includes Yarn and PostgreSQL 👌

[himal-mandalia]

In addition to Dockerising this app can we also update the REAMDE section on deploying to GOV.UK PaaS to reflect container based deploy.