Add docker documentation

[Arcelone]

Hi everyone,
I've found myself needing to create a local Mastodon instance for a Lab (something that's at test/demo level).
First of all, I browsed the documentation and the official repository and I saw that the debate on the need / desire to add documentation for docker.
However, as I haven't found a satisfactory tutorial or alternative on the Internet, I've taken the liberty of making a functional version, as detailed as possible, of what I'd like to find in the official documentation.
I'm sure it's not perfect, but if anyone would like to add or modify it, please don't hesitate.

PS: I didn't know what value to enter for "Weight", please feel free to modify it.