add instructions to build a quick docker image for dev and production

[Bilb]

What's left to be done is

• make sure the container restarts if it crashes (unsure if we can do this)
• for the production one, add a way to keep the sogs command running while the container is up
• deploy to dockerhub?

[jagerman]

Some minor changes included as comments here.

I also think we should move the /docker bits into /contrib/docker which might make people realize that this isn't for production use.

(There are some community members interested in creating a production docker image for pysogs, though that will look a little different — most likely by using the python3-sogs package to install the production version rather than reusing a current dev branch checkout.)