Support using customized Dockerfile for native image

[auke-]

Currently the Micronaut Maven plugin provides nice default Dockerfiles for a couple of native image variants and it allows the user to define its own customized Dockerfile. It is however not possible to provide a customized Dockerfile for native image creation and let the Micronaut Maven plugin add the build arguments like BASE_IMAGE and CLASS_NAME.

It would be helpful if the plugin uses a user provided Dockerfile when available and otherwise defaults to the build in Dockerfiles for native image creation.