Pass arguments to Dockerfile when you build it using --build-arg in order to have greater flexibility on docker build:

docker build -t essearch/ess-elasticsearch:1.7.6 --build-arg SHARDS_NO=5

If you want to pass multiple arguments to Dockerfile give another --build-arg and give your second argument as above.
Now initiate your args inside of Dockerfile as below:

ARG SHARDS_NO=""

#docker #container #build_arg #argument #build #image