Doc: Improve Docker download documentation with environments variables

[bkawakami]

Summary:
Adding environment variable information in Docker documentation. I really tried to understand how environment variables in Docker would work through this link #231, but I couldn't.

Link to documentation page:
https://www.focalboard.com/download/personal-edition/docker/

[romu70]

There is a bug, still not fixed about this. Here it is: #514