checkDockerImageExists
Checks whether a Docker Image exists either locally or remotely on Docker Hub.
This method will preferentially check for locally-existing images first, before resorting to Docker Hub.
Note that this method will return true if an error occurs while checking whether an image exists in Docker Hub. This is to allow config parsing and verification to continue, while deferring the errors to be emitted when the pipeline is actually executed.
Return
true if the image name/tag exists.
Parameters
imageName
The name of the image. Library images do NOT require library prepended to the front.
imageTag
The tag of the image.