Enable login via env vars (for CI)

[spkane]

Signed-off-by: Sean P. Kane sean@superorbital.io

- What I did

Borrowed code from docker#198 to make it possible to login via environment variables for CI workflows.

- How I did it

Via a few lines of Go code.

- How to verify it

Run hub-tool logout

export DOCKER_USERNAME=hub_username
export DOCKER_PASSWORD=hub_password
hub-tool login
hub-tool tag ls registry

• You should not be prompted for the username or password, and you should be able to run other commands now.

- Description for the changelog

• Allow non-interactive login via the DOCKER_USERNAME and DOCKER_PASSWORD env vars.