Clarification of docker section in README.md

[telenieko]

Fixes #128, the two distinct docker sections of the README are merged into one.

Checklist

• [x] I have enabled maintainer edits for this PR
• ~[ ] I have added tests for all changes in this PR~
• ~[ ] I have added documentation for the changes (in the manual)~
• ~[ ] There's a new file in changelog/unreleased/ that describes the changes for our users (template here)~
• ~[ ] I have run gofmt on the code in all commits~
• [x] All commit messages are formatted in the same style as the other commits in the repo
• [x] I'm done, this Pull Request is ready for review

[telenieko]

@MichaelEischer previous comments have been integrated into the proposed text, also integrated the merge conflict with the master branch.

If/When merging please squash commits to not fill master with the small changes