Add README.md file for the docker directory

See issue #5873

Docker-DCO-1.1-Signed-off-by: Timothy <timothyhobbs@seznam.cz> (github: https://github.com/timthelion)
use a numbered list for maintainers responsibility

At the moment maintainers responsibility are spelled out with a hybrid
list. I.e. an unordered list with an associated number.

In this commit I the number list proper is used.

Docker-DCO-1.1-Signed-off-by: Daan van Berkel <daan.v.berkel.1980@email.com> (github: dvberkel)
Fix go vet errors

Docker-DCO-1.1-Signed-off-by: Alexandr Morozov <lk4d4math@gmail.com> (github: LK4D4)

Docker-DCO-1.1-Signed-off-by: Victor Vieux <vieux@docker.com> (github: vieux)
Merge pull request #6495 from vieux/pr_6269

Fix go vet errors
Merge pull request #6399 from dvberkel/master

use a numbered list for maintainers responsibility
Merge pull request #5878 from timthelion/docs-docker-readme

Add README.md file for the docker directory