Support creating Docker image from local Flink distribution

Currently, https://github.com/apache/flink-docker/blob/dev-master/Dockerfile-debian.template only supports to create a Docker image from a Flink distribution which is hosted on a web server. I think it would be helpful if we could also create a Docker image from a Flink distribution which is stored on one's local file system. That way, one would not have to upload the file or start a web server for serving it.