The above answer by pl_rock is correct, the only thing I would add is to expect the ARG inside the Dockerfile if not you won't have access to it. So if you are doing
docker build -t essearch/ess-elasticsearch:1.7.6 --build-arg number_of_shards=5 --build-arg number_of_replicas=2 --no-cache .
Then inside the Dockerfile you should add
ARG number_of_replicas
ARG number_of_shards
I was running into this problem, so I hope I help someone (myself) in the future.