# 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](#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](env.md) 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](extends.md#example) and [reference](extends.md#reference). ### labels Add metadata to containers using [Docker labels](http://docs.docker.com/userguide/labels-custom-metadata/). 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](https://docs.docker.com/reference/logging/overview/)). 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](https://docs.docker.com/reference/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 (`$$`). ## Compose documentation - [User guide](/) - [Installing Compose](install.md) - [Get started with Django](django.md) - [Get started with Rails](rails.md) - [Get started with Wordpress](wordpress.md) - [Command line reference](/reference) - [Compose environment variables](env.md) - [Compose command line completion](completion.md)