Remove redundant docker file

[muuki88]

Expected behaviour

sbt docker:stage generates only one docker file.

Actual behaviour

Two files are generated:

tree target/docker
target/docker/
├── Dockerfile <---- here
└── stage
    ├── Dockerfile  <---- here
    └── opt
        └── docker
            ├── bin
            │   ├── docker-test
            │   └── docker-test.bat
            └── lib
                ├── docker-test.docker-test-0.1.0.jar

This causes a lot of confusion as in #1006