Answer common Docker (+ docker-compose) questions

[begriffs]

Why are we implementing it? (sales eng)

What are the typical use cases?

• How can you run multiple Citus clusters at once?

Samay can share more about what the most common questions are. As long as they pertain to Citus and aren't merely basic Docker usage questions, we should include the answers in docs.

Communication goals (e.g. detailed howto vs orientation)

Good locations for content in docs structure

Adding to https://docs.citusdata.com/en/v7.1/installation/single_machine_docker.html might not match the pages for other methods of installation, so we could make a new section? Maybe inside the Administration section.

[begriffs]

@saicitus could you update me with what you think the most common questions are? Then I'll research the answers and write them up.

[colton-citus]

1. How to create persistent volumes? See https://github.com/citusdata/membership-manager/issues/4
2. How to change language? See https://github.com/citusdata/docker/issues/90
3. How to add extensions (pg_partman was specific question, this should be generalized) See https://github.com/citusdata/wiki/wiki/Unusual-Docker-configurations
4. Citus version on Docker isn't very up to date. More of a comment...