Replace image and tag inputs with a single image input

[shrink]

The use of separate image and tag inputs which are then assembled is awkward because an "image" is identified by a combination of image name and image tag (no explicit tag implicitly means latest). A user should be able to specify a single image input which is a complete name:tag image identifier because that is the output of many other tools.

A tag is a label applied to a Docker image in a repository. Tags are how various images in a repository are distinguished from each other.