How to handle IP addresses when linking docker con

I am using docker-compose to build a complete development stack.

The application needs a mysql server to work.

The mysql server is an external container setup by docker-compose:

mysql:
    image: mysql:5.6
    volumes:
        - /data/mysql:/var/lib/mysql
        - ./docker/mysql.d:/etc/mysql/conf.d
    ports:
        - "3306:3306"
    environment:
        MYSQL_ROOT_PASSWORD: password

The application has its own docker-compose.yml and references the mysql container:

my-application:
    build: . # the Dockerfile resides in the current folder
    ports:
        - "9180:80"
        - "9543:443"
    external_links:
        - mysql_mysql_1:mysql
environment:
    DOCKER_ENVIRONMENT: dev
    DB_NAME: local_db
    DB_PASS: password
    DB_USER: root
    DB_HOST: # how to set the mysql's IP address?

I cannot pass them in the docker-compose as it is dynamic.

I know that the application is aware of the mysql IP address, as I have certain variables set:

application-container$ env|grep ADDR 
    MYSQL_PORT_3306_TCP_ADDR=172.17.0.241

Yet this is not my required DB_HOST.

Can I map the variable somehow to DB_HOST or set it differently?

You don't have to set the IP, but you can reference the container's virtual hostname, and this is the same value as you named your linked container.

This means you can indeed set the DB_HOST from within the docker-compose.yml, either with links (recommended) or external_links:

your_application:
    build: .
    ports:
        - "9180:80"
        - "9543:443"
    external_links:
        - mysql_mysql_1:docker-mysql
    environment:
        DB_HOST: docker-mysql

As when you connect to your docker container, you could connect to your mysql container:

application-container $ mysql -h docker-mysql -uroot -ppassword -p 3360

It works the same when you link container's from the same docker-composer.yml as well.

This is also documented:

Link to containers in another service. Either specify both the service name and the link alias (SERVICE:ALIAS), or just the service name (which will also be used for the alias).
links:
 - db
 - db:database
 - redis
An entry with the alias' name will be created in /etc/hosts inside containers for this service, e.g:
172.17.2.186  db
172.17.2.186  database
172.17.2.187  redis
Environment variables will also be created - see the environment variable reference for details.