Fixing command line elements to be able to use digest style image tags

[kellrott]

Updating behavior for dealing with tags that use the digest format (https://docs.docker.com/engine/reference/commandline/images/#list-image-digests)

Example:

udocker pull ubuntu@sha256:7c9c7fed23def3653a0da5bc9ecb651efe155ebd5802c7ba5d585edaa6c89496

Previously, this would have failed because the code would not accept the @ character. Split behavior/join also changes, using the @ rather then the :.

Previously this would have failed. For some tools, such as Cromwell using the digest form is considered to be best practice