Add documentation for docker deployment

[jonoliver]

Add "Docker" section to our deployment docs, similar to our Pantheon deployment docs.

• Capture any Github secrets / variables that need to be set in order for the workflow to run
• Add documentation around how to run a container for the image. See these notes. Note that the instructions in that comment are specific to running locally. The documentation should be more broadly focused on how to run the container in a production environment.