Document Docker Setup

[niccokunzmann]

In the pixelfed Matrix Chat, there was a discussion about setting up Pixelfed with Docker.
I personally like this choice.
There seem to be official images for that.

So, the documentation should answer:

• How can we set up pixelfed with docker?
• What are the required steps?
• Who would like to review/create such a contribution because they use it?
• Which images can be used? Related Work section for the images of pixelfed.de

Related Issues:

• Docker install instructions? #40
• official Docker image #19

See also:

• https://kushaldas.in/posts/pixelfed-on-docker.html
• https://pimylifeup.com/raspberry-pi-pixelfed/