doc update: docker-compose-override.yaml

[Shelton]

Per some conversations, we need to elaborate on docker-compose-override.yml. Thoughts on this started with @christophermaier, and @vanstee just mentioned that he might have some content written up about docker-compose-override.yml.

[vanstee]

I briefly explained how this worked in the new installation guide. @Shelton do you know if there was anything specific needed here?