Fix typo
Since #13463 support for previous version of docker-py older than 0.3.2 is broken

Reflect this change in documentation
docker-py==0.3.2 already reads '~/.dockercfg'

So drop its support

Fixes #13305
docker api > 1.9
Merge pull request #13690 from ticosax/docker=1.0

Docker=1.0