Improve docker documentation

[alekzvik]

• Move out docker docs to a separate file.
• Add an explanation of how it works internally
• Add FAQ section

Interest was expressed here: #511

[alekzvik]

@m-mohr since you first expressed interest in this doc, please take a look and let me know what is missing and what can be improved

[m-mohr]

Thanks, I appreciate it. I left a couple of editorial suggestions.

Generally, I think we could move the whole Docker section from the README to the docker.md and just leave the header and a link to the docs in the README. That would slim down the README a bit. But just an idea, not a must.

[alekzvik]

I agree with your suggestion to move out the docker section.
I yanked it in full in this revision and left a small note with a link in README.
Other editorial changes are also fixed.

[m-mohr]

Thank you very much!

[alekzvik]

@m-mohr Thank you for all the corrections and editing.