More on the Docker Compose File

Attach persistent Docker volumes

Mount a Docker volume (created on first use) or bind a host directory:

  mycontainer:
    volumes:

      #Docker volume
      - type: volume
        source: rootfiles
        target: /var/www/html

      #bind host directory
      - type: bind
        source: ./myfiles
        target: /var/www/html/myfiles

A shorter syntax can also be used to define <source>:<destination>. The <source> is presumed to be Docker volume unless it starts with . or .. relative file paths.

  mycontainer:

    #idential to above
    volumes:
      - rootfiles:/var/www/html
      - ./myfiles:/var/www/html/myfiles

Docker volumes (and optional configurations) must be referenced after the services: definition at the bottom of docker-compose.yml:

volumes:
  rootfiles:

📌 The containers you specify in your Compose file are under your services tag.

Set a custom dns server

Define one or more DNS servers:

  mycontainer:
    dns:
      - 1.1.1.1
      - 8.8.8.8

expose ports

Ports can be exposed to the host using “host:container” notation:

  mycontainer:
    expose:
      - "3000:3000"

Using a single container value, such as:

  mycontainer:
    expose:
      - "3000"

It only exposes the port to other containers on the same Docker network and not to the host this file is running on.

More than one port can be exposed for a container and even to a different port, e.g.,

  nodejs:
    expose:
      - "8000:3000"
      - "9229:9229"

The default port 3000 of the container or service nodejs is mapped on 8000 so now it’s available on 8000. Moreover, if you want to use devTools for Node.js debugging , you need to expose the 9229 port to 9229, which can be done by adding another port under the expose property.

Define external_links to other containers

Containers started outside the current docker-compose.yml file can be referenced assuming they are on the same Docker network:

  mycontainer:
    external_links:
      - mysql

Override the defaults

Override the Dockerfile CMD command using shell or exec (array) forms:

  mycontainer:
    command: node ./test.js
    #or [ "node", "./test.js" ]

Override the Dockerfile ENTRYPOINT command using shell or exec (array) forms:

  mycontainer:
    entrypoint: node ./test.js
    #or [ "node", "./test.js" ]

Specify a restart policy

Restart policies include:

• no: never restart the container (the default)
• always: always restart the container when it stops
• on-failure: restart the container when it fails with an exit code

  mycontainer:
    restart: on-failure

Run a healthcheck

Configure a check that is run periodically to check a container is alive and responding:

  mycontainer:
    healthcheck:
      test: [ "CMD", "curl", "-f", "http://localhost:3000/test" ]
      interval: 1m00s
      timeout: 10s
      retries: 3

Define a logging service

A log can be output using the driver of "json-file", "syslog", or "none":

  mycontainer:
    logging:
      driver: syslog
      options:
        syslog-address: "tcp://192.168.1.100:1234"

The option "none" does not display logs on the console. For both options, "syslog" and "json-file", the log-driver and log-opts keys need to be set with appropriate values in the daemon.json.