Below is an example of how to setup gnatsd cluster using Docker.

I have put 3 different configurations (one per gnatsd server) under a folder named conf as follows:

tree conf

conf
|-- gnatsd-A.conf
|-- gnatsd-B.conf
`-- gnatsd-C.conf

Each one of those files have the following content below: (Here I am using ip 192.168.59.103 as an example, so just replace with the proper ip from your server)

# Copyright 2012-2015 Apcera Inc. All rights reserved.

# Cluster Server A

port: 7222

cluster {
  host: '0.0.0.0'
  port: 7244

  routes = [
    nats-route://192.168.59.103:7246
    nats-route://192.168.59.103:7248
  ]
}

# Copyright 2012-2015 Apcera Inc. All rights reserved.

# Cluster Server B

port: 8222

cluster {
  host: '0.0.0.0'
  port: 7246

  routes = [
    nats-route://192.168.59.103:7244
    nats-route://192.168.59.103:7248
  ]
}

# Copyright 2012-2015 Apcera Inc. All rights reserved.

# Cluster Server C

port: 9222

cluster {
  host: '0.0.0.0'
  port: 7248

  routes = [
    nats-route://192.168.59.103:7244
    nats-route://192.168.59.103:7246
  ]
}

Then on each one of your servers, yo should be able to start the gnatsd image as follows:

docker run -it -p 0.0.0.0:7222:7222 -p 0.0.0.0:7244:7244 --rm -v $(pwd)/conf/gnatsd-A.conf:/tmp/cluster.conf apcera/gnatsd -c /tmp/cluster.conf -p 7222 -D -V

docker run -it -p 0.0.0.0:8222:8222 -p 0.0.0.0:7246:7246 --rm -v $(pwd)/conf/gnatsd-B.conf:/tmp/cluster.conf apcera/gnatsd -c /tmp/cluster.conf -p 8222 -D -V

docker run -it -p 0.0.0.0:9222:9222 -p 0.0.0.0:7248:7248 --rm -v $(pwd)/conf/gnatsd-C.conf:/tmp/cluster.conf apcera/gnatsd -c /tmp/cluster.conf -p 9222 -D -V