We figured out to use the solution with the ambassador. It is indeed the more comfortable solution.
The configuration that works for us:
amba1:
  image: cpuguy83/docker-grand-ambassador
  volumes:
    - "/var/run/docker.sock:/var/run/docker.sock"
  command: "-name cucumber_dbreplicasetpart1_1"
amba2:
  image: cpuguy83/docker-grand-ambassador
  volumes:
    - "/var/run/docker.sock:/var/run/docker.sock"
  command: "-name cucumber_dbreplicasetpart2_1"
amba3:
  image: cpuguy83/docker-grand-ambassador
  volumes:
    - "/var/run/docker.sock:/var/run/docker.sock"
  command: "-name cucumber_dbreplicasetpart3_1"
dbreplicasetpart1:
  image: 'mongo:2.6.8'
  hostname: dbreplicasetpart1
  command: >
    bash -c
    '
      mongod --fork --logpath mongo.log --smallfiles --replSet rs1
      echo "
        printjson(
          rs.initiate(
            {
              _id : \"rs1\",
              members : [
                {_id : 0, host : \"dbreplicasetpart1:27017\"},
                {_id : 1, host : \"dbreplicasetpart2:27017\"},
                {_id : 2, host : \"dbreplicasetpart3:27017\"},
              ]
            }
          )
        );
      " | mongo;
      tail -f mongo.log
    '
  links:
    - "amba2:dbreplicasetpart2"
    - "amba3:dbreplicasetpart3"
dbreplicasetpart2:
  image: 'mongo:2.6.8'
  hostname: dbreplicasetpart2
  command: >
    bash -c
    '
      mongod --fork --logpath mongo.log --smallfiles --replSet rs1
      echo "
        printjson(
          rs.initiate(
            {
              _id : \"rs1\",
              members : [
                {_id : 0, host : \"dbreplicasetpart1:27017\"},
                {_id : 1, host : \"dbreplicasetpart2:27017\"},
                {_id : 2, host : \"dbreplicasetpart3:27017\"},
              ]
            }
          )
        );
      " | mongo;
      tail -f mongo.log
    '
  links:
    - "amba1:dbreplicasetpart1"
    - "amba3:dbreplicasetpart3"
dbreplicasetpart3:
  image: 'mongo:2.6.8'
  hostname: dbreplicasetpart3
  command: >
    bash -c
    '
      mongod --fork --logpath mongo.log --smallfiles --replSet rs1
      echo "
        printjson(
          rs.initiate(
            {
              _id : \"rs1\",
              members : [
                {_id : 0, host : \"dbreplicasetpart1:27017\"},
                {_id : 1, host : \"dbreplicasetpart2:27017\"},
                {_id : 2, host : \"dbreplicasetpart3:27017\"},
              ]
            }
          )
        );
      " | mongo;
      tail -f mongo.log
    '
  links:
    - "amba1:dbreplicasetpart1"
    - "amba2:dbreplicasetpart2"