Allow latest tag other than "latest" to be specified

In active pipelines with different versions it is convenient to have tags such as 1.0-latest, and 2.0-latest, propose a simple change to specify a latest tag instead of hard coded in "latest". There is a branch with changes on it in https://github.com/larrype/docker-image-resource and a working version at https://hub.docker.com/r/p41159/docker-image-resource/.

Note: the first solution used solve this was to accept a list of tags and/or list of file paths to tags. This worked as well but seems more significant of a change. It also would use the existence of a file to determine if it was a tag or a file path to a file containing a tag. It was much more flexible but simply allowing the latest tag to be satisfied and was a minimal change that satisfied the need. If interested, here is the tag list enhancement... https://github.com/larrype/docker-image-resource/tree/support-tag-list

Also, I did see #124 and the associated pull request which also would seem to solve the problem - although i would prefer to support a list of tags or list of files each containing a single tag rather than reading a list of tags out of a single file (if I read that PR properly)

Start by reviewing the docker-image-resource repository and the linked larrype/docker-image-resource branch, then compare the working Docker Hub version with the current behavior. Confirm how the resource handles the hard-coded "latest" tag and define done as allowing a configured version-specific latest tag, such as 1.0-latest or 2.0-latest, without requiring the larger tag-list enhancement.