Basic auth support

[vito]

Right now there's no way to configure auth, which means this resource is only useful for fetching public images.

There's all sorts of auth we'd need to support various registries. Let's start with the simplest one: basic auth.

[marco-m]

If I understand correctly now this resource supports basic auth, for example in https://github.com/concourse/concourse/blob/e5d5b9af1ef45243fe9062a86fe7392e74cd5f6a/ci/pipelines/concourse.yml#L714

- name: unit-image
  type: registry-image
  source:
    repository: concourse/unit
    username: ((docker.username))
    password: ((docker.password))	

is this correct ?

[vito]

Right now it's only supported by put because I needed it for pushing to work at all. So check and get still can't use basic auth for private images.

[int-tt]

Thank you for a wonderful resource!
I needed to use a private repository in my work, so I tried writing PR:) #9

[vito]

finished, tested, and documented!

thanks for the PR, @int-tt!