add cfssljson to docker image

[Victor Login]

It would be nice to add cfssljson to the docker image cfssl/cfssl

[Nicky Semenza]

all the binaries are there, you simply need to override the entrypoint

docker run --entrypoint cfssljson cfssl/cfssl --help
Usage of cfssljson:
  -bare
    	the response from CFSSL is not wrapped in the API standard response