Docker + Marathon : how to do a port mapping ?
The port mapping is (as with Docker) only relevant for BRIDGE networking mode, see also the Marathon doc. In your case the following should work:
..."container": { "type": "DOCKER", "docker": { "network": "BRIDGE", "portMappings": [ { "protocol": "tcp", "containerPort": 8787, "hostPort": 8787 } ] }},...