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docker-compose.yml reference

Each service defined in docker-compose.yml must specify exactly one of image or build. Other keys are optional, and are analogous to their docker run command-line counterparts.

As with docker run, options specified in the Dockerfile (e.g., CMD, EXPOSE, VOLUME, ENV) are respected by default - you don't need to specify them again in docker-compose.yml.

Values for configuration options can contain environment variables, e.g. image: postgres:${POSTGRES_VERSION}. For more details, see the section on variable substitution.

image

Tag, partial image ID or digest. Can be local or remote - Compose will attempt to pull if it doesn't exist locally.

image: ubuntu
image: orchardup/postgresql
image: a4bc65fd
image: busybox@sha256:38a203e1986cf79639cfb9b2e1d6e773de84002feea2d4eb006b52004ee8502d

Using image together with either build or dockerfile is not allowed. Attempting to do so results in an error.

build

Path to a directory containing a Dockerfile. When the value supplied is a relative path, it is interpreted as relative to the location of the yml file itself. This directory is also the build context that is sent to the Docker daemon.

Compose will build and tag it with a generated name, and use that image thereafter.

build: /path/to/build/dir

Using build together with image is not allowed. Attempting to do so results in an error.

dockerfile

Alternate Dockerfile.

Compose will use an alternate file to build with.

dockerfile: Dockerfile-alternate

Using dockerfile together with image is not allowed. Attempting to do so results in an error.

command

Override the default command.

command: bundle exec thin -p 3000

links

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.

external_links

Link to containers started outside this docker-compose.yml or even outside of Compose, especially for containers that provide shared or common services. external_links follow semantics similar to links when specifying both the container name and the link alias (CONTAINER:ALIAS).

external_links:
 - redis_1
 - project_db_1:mysql
 - project_db_1:postgresql

extra_hosts

Add hostname mappings. Use the same values as the docker client --add-host parameter.

extra_hosts:
 - "somehost:162.242.195.82"
 - "otherhost:50.31.209.229"

An entry with the ip address and hostname will be created in /etc/hosts inside containers for this service, e.g:

162.242.195.82  somehost
50.31.209.229   otherhost

ports

Makes an exposed port accessible on a host and the port is available to any client that can reach that host. Docker binds the exposed port to a random port on the host within an ephemeral port range defined by /proc/sys/net/ipv4/ip_local_port_range. You can also map to a specific port or range of ports.

Acceptable formats for the ports value are:

containerPort
ip:hostPort:containerPort
ip::containerPort
hostPort:containerPort

You can specify a range for both the hostPort and the containerPort values. When specifying ranges, the container port values in the range must match the number of host port values in the range, for example, 1234-1236:1234-1236/tcp. Once a host is running, use the 'docker-compose port' command to see the actual mapping.

The following configuration shows examples of the port formats in use:

ports:
 - "3000"
 - "3000-3005"
 - "8000:8000"
 - "9090-9091:8080-8081"
 - "49100:22"
 - "127.0.0.1:8001:8001"
 - "127.0.0.1:5000-5010:5000-5010"

When mapping ports, in the hostPort:containerPort format, you may experience erroneous results when using a container port lower than 60. This happens because YAML parses numbers in the format xx:yy as sexagesimal (base 60). To avoid this problem, always explicitly specify your port mappings as strings.

expose

Expose ports without publishing them to the host machine - they'll only be accessible to linked services. Only the internal port can be specified.

expose:
 - "3000"
 - "8000"

volumes

Mount paths as volumes, optionally specifying a path on the host machine (HOST:CONTAINER), or an access mode (HOST:CONTAINER:ro).

volumes:
 - /var/lib/mysql
 - ./cache:/tmp/cache
 - ~/configs:/etc/configs/:ro

You can mount a relative path on the host, which will expand relative to the directory of the Compose configuration file being used. Relative paths should always begin with . or ...

Note: No path expansion will be done if you have also specified a volume_driver.

volumes_from

Mount all of the volumes from another service or container.

volumes_from:
 - service_name
 - container_name

environment

Add environment variables. You can use either an array or a dictionary. Any boolean values; true, false, yes no, need to be enclosed in quotes to ensure they are not converted to True or False by the YML parser.

Environment variables with only a key are resolved to their values on the machine Compose is running on, which can be helpful for secret or host-specific values.

environment:
  RACK_ENV: development
  SHOW: 'true'
  SESSION_SECRET:

environment:
  - RACK_ENV=development
  - SHOW=true
  - SESSION_SECRET

env_file

Add environment variables from a file. Can be a single value or a list.

If you have specified a Compose file with docker-compose -f FILE, paths in env_file are relative to the directory that file is in.

Environment variables specified in environment override these values.

env_file: .env

env_file:
  - ./common.env
  - ./apps/web.env
  - /opt/secrets.env

Compose expects each line in an env file to be in VAR=VAL format. Lines beginning with # (i.e. comments) are ignored, as are blank lines.

# Set Rails/Rack environment
RACK_ENV=development

extends

Extend another service, in the current file or another, optionally overriding configuration.

Here's a simple example. Suppose we have 2 files - common.yml and development.yml. We can use extends to define a service in development.yml which uses configuration defined in common.yml:

common.yml

webapp:
  build: ./webapp
  environment:
    - DEBUG=false
    - SEND_EMAILS=false

development.yml

web:
  extends:
    file: common.yml
    service: webapp
  ports:
    - "8000:8000"
  links:
    - db
  environment:
    - DEBUG=true
db:
  image: postgres

Here, the web service in development.yml inherits the configuration of the webapp service in common.yml - the build and environment keys - and adds ports and links configuration. It overrides one of the defined environment variables (DEBUG) with a new value, and the other one (SEND_EMAILS) is left untouched.

The file key is optional, if it is not set then Compose will look for the service within the current file.

For more on extends, see the tutorial and reference.

labels

Add metadata to containers using Docker labels. You can use either an array or a dictionary.

It's recommended that you use reverse-DNS notation to prevent your labels from conflicting with those used by other software.

labels:
  com.example.description: "Accounting webapp"
  com.example.department: "Finance"
  com.example.label-with-empty-value: ""

labels:
  - "com.example.description=Accounting webapp"
  - "com.example.department=Finance"
  - "com.example.label-with-empty-value"

container_name

Specify a custom container name, rather than a generated default name.

container_name: my-web-container

Because Docker container names must be unique, you cannot scale a service beyond 1 container if you have specified a custom name. Attempting to do so results in an error.

log_driver

Specify a logging driver for the service's containers, as with the --log-driver option for docker run (documented here).

The default value is json-file.

log_driver: "json-file"
log_driver: "syslog"
log_driver: "none"

Note: Only the json-file driver makes the logs available directly from docker-compose up and docker-compose logs. Using any other driver will not print any logs.

log_opt

Specify logging options with log_opt for the logging driver, as with the --log-opt option for docker run.

Logging options are key value pairs. An example of syslog options:

log_driver: "syslog"
log_opt:
  syslog-address: "tcp://192.168.0.42:123"

net

Networking mode. Use the same values as the docker client --net parameter.

net: "bridge"
net: "none"
net: "container:[name or id]"
net: "host"

pid

pid: "host"

Sets the PID mode to the host PID mode. This turns on sharing between container and the host operating system the PID address space. Containers launched with this flag will be able to access and manipulate other containers in the bare-metal machine's namespace and vise-versa.

dns

Custom DNS servers. Can be a single value or a list.

dns: 8.8.8.8
dns:
  - 8.8.8.8
  - 9.9.9.9

cap_add, cap_drop

Add or drop container capabilities. See man 7 capabilities for a full list.

cap_add:
  - ALL

cap_drop:
  - NET_ADMIN
  - SYS_ADMIN

dns_search

Custom DNS search domains. Can be a single value or a list.

dns_search: example.com
dns_search:
  - dc1.example.com
  - dc2.example.com

devices

List of device mappings. Uses the same format as the --device docker client create option.

devices:
  - "/dev/ttyUSB0:/dev/ttyUSB0"

security_opt

Override the default labeling scheme for each container.

  security_opt:
    - label:user:USER
    - label:role:ROLE

working_dir, entrypoint, user, hostname, domainname, mac_address, mem_limit, memswap_limit, privileged, restart, stdin_open, tty, cpu_shares, cpuset, read_only, volume_driver

Each of these is a single value, analogous to its docker run counterpart.

cpu_shares: 73
cpuset: 0,1

working_dir: /code
entrypoint: /code/entrypoint.sh
user: postgresql

hostname: foo
domainname: foo.com

mac_address: 02:42:ac:11:65:43

mem_limit: 1000000000
memswap_limit: 2000000000
privileged: true

restart: always

stdin_open: true
tty: true
read_only: true

volume_driver: mydriver

Variable substitution

Your configuration options can contain environment variables. Compose uses the variable values from the shell environment in which docker-compose is run. For example, suppose the shell contains POSTGRES_VERSION=9.3 and you supply this configuration:

db:
  image: "postgres:${POSTGRES_VERSION}"

When you run docker-compose up with this configuration, Compose looks for the POSTGRES_VERSION environment variable in the shell and substitutes its value in. For this example, Compose resolves the image to postgres:9.3 before running the configuration.

If an environment variable is not set, Compose substitutes with an empty string. In the example above, if POSTGRES_VERSION is not set, the value for the image option is postgres:.

Both $VARIABLE and ${VARIABLE} syntax are supported. Extended shell-style features, such as ${VARIABLE-default} and ${VARIABLE/foo/bar}, are not supported.

If you need to put a literal dollar sign in a configuration value, use a double dollar sign ($$).

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