Store credentials for multiple accounts #326

I have multiple .docker config dirs to manage different DockerHub accounts across projects. I would like to use the secretservice manager, but it shares one login across all the docker configs.

This might be simple enough to implement if the key for the stored credential was https://index.docker.io/v1/:$USERNAME instead of just https://index.docker.io/v1/.

In which case, the ./docker/config.json entry would probably need to look more like this:

{
  "credsStore": "osxkeychain",
  "username": "blah"
}

So that docker-credentials-xxx knows which credential to pull.