Compatibility with gradle-docker-plugin 6.0.0

[Former user]

See https://twitter.com/bmuschko/status/1196077894335729665?s=20: version 6.0.0 of the gradle-docker-plugin was released, but this plugin is not compatible. I am willing to spend time on this myself, but I wasn't sure if this project is still maintained which is why I'm creating an issue first: please let me know if you'll consider a merge request to fix this, and if you would be OK with dropping compatibility with older versions of that plugin.

[Joris Kuipers]

Sorry, just realized I wasn’t logged in. This was me

[Patrick Double]

Yes, I’m planning to keep this plugin maintained. A pull request is appreciated!

[Patrick Double]

Release 0.6.0