Multiple registry credentials broken since v9.1.0 release and code convert to Java

[kleini]

With commit 59eec98e7e4c7f24dbc8aa247ef5347c90125de8 the RegistryAuthLocator constructors are made private. This breaks the possibility to subclass the RegistryAuthLocator with v9.1.0 release. Our Dockerfile pulls images from different registries and therefore we need to provide multiple credentials for the DockerBuildImage task. Your plugins extension and your tasks implementation only allows to register a single credential.