Remove the Docker Gradle plugin dependency and solr/docker/package module

Currently Solr uses the gradle docker plugin to manage building Solr docker images. When migrating the docker build into Solr, using the plugin was the path of least resistance and allowed us to migrate without having to think a lot about the gradle part.

Now that the docker image building works, it may be beneficial to migrate away from the docker plugin, so that we can have better control over build process. The steps are simple enough anyways, so we wouldn't be sacrificing much to have more flexibility.

Given the discussion in the community over release vs local docker image builds, there does not seem to be a desire to have separate builds for them. Therefore we no longer need the solr/docker/package module. Instead the solr/docker module will pull in the solr/packaging output and directly put it into the docker image.