This commit fixes the missing pieces in our Maven Central publication
pipeline. Our first attempt at releasing with it showed a few problems:
* The promote task did not have the artifacts downloaded with the
artifactory repository.
* We applied the wrong Sonatype credentials to the task.
* The github changelog task would fail because of docker rate limiting
since we were not using the right type of resource, which is
configured with the proper caching mechanism.
See gh-1199
