Adding an optional parameter to override the exported variable name

[jfourment]

Hi,

We have been successfilly using this plugin with the docker image plugin.
We also have our in-house plugin that build images and do a bunch of other things for us, and I'd like to be able to chain this cache plugin with our plugin.
For that to work, we would need to be able to override the name of the exported variable containing the image name and tag.

I forked the repo and have a working change I'm happy to merge into the main repo.

Of course, the change would be backward compatible and the variable name would still default to BUILDKITE_PLUGIN_DOCKER_IMAGE