Add Docker setup to official documentation

[melroy89]

Add a Docker section to the official documentation.

Explain the setup using Docker compose and how to configure the .env correctly, etc.

During upgrades like v4.3, explain to people they need to change the docker image to: image: ghcr.io/mastodon/mastodon-streaming:v4.3 and the command to: command: node ./streaming/index.js. The latter is also missing.

Original issue ticket: mastodon/mastodon#32437

[Arcelone]

I already did some work here : #1446 but no news since 6 months.
Feel free to add things if needed.
Would be happy to work on that 😄

[mjankowski]

I closed a bunch of Docker-related issues and linked them all here. We should attempt to include/resolve as much of that as we can in a Docker PR, which I hereby nominate to be #1446