Mark "native-k8s" as deprecated in docker-entrypoint.sh

When we are publishing the image 1.12 to docker hub, some docker guys raise up a issue for the docker-entrypoint.sh. They want the images to have a certain standard, because they are the official ones. However the proposed native-k8s command is more like an internal bridge. It is only used for native Kubernetes integration.

Another suggestion is removing the "bash -c" wrapper and generate it in the flink codes. Refer here[1] for more information.

Note: We mark the native-k8s as deprecated and export the environments for all pass-through mode commands, the flink Kubernetes codes should be adjusted accordingly.

[1]. https://github.com/docker-library/official-images/pull/9249