Refactor completion for docker run and docker create

_docker_run and _docker_create had only one differing line.

This refactoring features:

- direct completion for both commands to the same function
- factor out the common arguments, sort & format them nicely
- compute the argument for _docker_pos_first_nonflag.

Signed-off-by: Harald Albers <github@albersweb.de>
Merge pull request #9671 from albers/refactor-create-and-run

Refactor completion for docker run and docker create