Make automatic project version tag optional

[fletch3555]

What happened?

When building an image, this plugin will automatically append the project version to the list of tags (ref: https://github.com/palantir/gradle-docker/blob/develop/src/main/groovy/com/palantir/gradle/docker/DockerExtension.groovy#L98).

What did you want to happen?

I plan to add tags conditionally, so I need this to not happen automatically, or at least a way to disable it.

I would suggest the best user experience would be for the ONLY automatic tag to be the one matching the required "name" property, whether that's implicit ("latest") or provided explicitly.

I'll gladly open a PR for this if there's consensus. Thanks!

[fletch3555]

Apologies, this is a duplicate of #516. Not sure how I didn't see it before.