create a new network like this:
$ docker network create rmnet --subnet=172.19.0.0/16 --gateway=172.19.0.1

version: '3.1'

networks:
  rmnet:
    external: true

services:
  mysqldb:
    image: mysql:5.6
    ports:
      - 3306:3306
    restart: always
    networks:
      rmnet:
        ipv4_address: 172.19.0.4
        aliases:
          - mysql-server
    environment:
      MYSQL_DATABASE: "database_name"
      MYSQL_USER: "user_name"
      MYSQL_PASSWORD: "password"
      MYSQL_ROOT_PASSWORD: "root_user_password"

  kms:
    image: kurento/kurento-media-server:trusty-latest
    ports:
      - 8888:8888
    networks:
      rmnet:
        ipv4_address: 172.19.0.5
        aliases:
          - kms

  nginx:
    image: nginx
    ports:
      - 8080:80
      - 8443:443
    volumes:
      - ./nginx.conf:/etc/nginx/nginx.conf
      - ./nginx.crt:/opt/certs/nginx.crt
      - ./nginx.key:/opt/certs/nginx.key
    networks:
      rmnet:
        ipv4_address: 172.19.0.6
        aliases:
          - nginx-server