Support upstream-resources hosted in private repositories

[arturo-seijas]

Publishing a charm specifying an uptream resource hosted on a private repository that requires authentication is not currently supported. Specifically, I'm trying to use an image uploaded to charmhub

upstream-source: 'registry.jujucharms.com/charm/e7cgxprwp5telxhlb24qmbxuoi9a5e3m40iv/indico-image@sha256:c40b5291a0346132d1f6ed5a38f92fafdefd7da64655742881729a8cbaf6a807'

getting the following error message:

Error response from daemon: pull access denied for undefined, repository does not exist or may require 'docker login': denied: requested access to the resource is denied

[simskij]

Discussed this with Arturo and I think it makes an interesting addition to the action. It requires us to add input for docker credentials, and pass that on to docker when we try to pull the image.