supported various options when docker run

[tkuchiki]

Changed CMD to ENTRYPOINT in Dockerfile.

Results are as follows:

$ docker build -t mykatsubushi ./docker

# /usr/local/bin/katsubushi -worker-id 1
$ docker run -t mykatsubushi -worker-id 1

# /usr/local/bin/katsubushi -redis redis://localhost:6379/0
$ docker run -t mykatsubushi -redis redis://localhost:6379/0

ref: #31