Infer `latest` tag when no tag is specified on a user-provided base image

[baronfel]

For usability, we should infer a tag of latest when a user provides a base image with no tag. This matches the Docker CLI tooling and should provide an easier onboarding story. We should issue a warning when doing this as well, something like No tag was specified for {image registry and repository}, so 'latest' is being used as the tag..