Podman and docker: dynamically skip registry login step

[dometto]

Request

Make it possible to use an environment variable to determine whether the registry login step should be skipped or not. Presently this is impossible, because the conditional for that step look as follows:

# docker and podman create.yml
      when:
        - item.registry is defined
        - item.registry.credentials is defined
        - item.registry.credentials.username is defined

This means that if the registry key is defined, regardless of what the values to any of its keys are, the login step is nevertheless performed. So this:

---
platforms:
  - name: name
    image: ghcr.io/foo/bar:latest
    pre_build_image: true
    registry:
      url: $DOCKER_REGISTRY
      credentials:
        username: $DOCKER_USER
        password: $DOCKER_PW

...means the login step is performed, even if all the $DOCKER_ variables are unset.

Use case

I would like for molecule to exhibit the following behaviour:

1. If we're on CI, login and pull the image
2. If we're running molecule locally, don't login and use local images

This is because on CI, I can use a token to login to the container registry, but I don't want my teammembers to have to login to the registry when running tests locally.

So something like the platforms definition provided above should perform the login step if and only if the $DOCKER_ environment variables are set, and skip it otherwise.

Proposal

The login step should be skipped if the url subkey of registry is empty/None. If the user has set the registry to an empty string (or in my example case, has not set the $DOCKER_REGISTRY env variable), that seems a clear signal that they do not want to engage with the registry in any way.

Alternatively, the login step could also be skipped if the pull key is explicitly set to false by the user.