Using Linking ENV Variables into Run command

How can I use a link env variables in the run command?
I need to run following command: /nsqd -lookupd-tcp-address $NSQLOOKUP_PORT

UPDATE : solved, I just hardcode the value.

Just to follow up, were you trying to do this in Rancher or RancherOS? Also, have you looked into the rancher-compose section on environment interpolation.