Add "disambiguation" page for environment variables

Signed-off-by: Aanand Prasad <aanand.prasad@gmail.com>

docs/environment-variables.md

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+<!--[metadata]>
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+title = "Environment variables in Compose"
+description = "How to set, use and manage environment variables in Compose"
+keywords = ["fig, composition, compose, docker, orchestration, environment, variables, env file"]
+[menu.main]
+parent = "workw_compose"
+weight=10
++++
+<![end-metadata]-->
+
+# Environment variables in Compose
+
+There are multiple parts of Compose that deal with environment variables in one sense or another. This page should help you find the information you need.
+
+
+## Substituting environment variables in Compose files
+
+It's possible to use environment variables in your shell to populate values inside a Compose file:
+
+    web:
+      image: "webapp:${TAG}"
+
+For more information, see the [Variable substitution](compose-file.md#variable-substitution) section in the Compose file reference.
+
+
+## Setting environment variables in containers
+
+You can set environment variables in a service's containers with the ['environment' key](compose-file.md#environment), just like with `docker run -e VARIABLE=VALUE ...`:
+
+    web:
+      environment:
+        - DEBUG=1
+
+
+## Passing environment variables through to containers
+
+You can pass environment variables from your shell straight through to a service's containers with the ['environment' key](compose-file.md#environment) by not giving them a value, just like with `docker run -e VARIABLE ...`:
+
+    web:
+      environment:
+        - DEBUG
+
+The value of the `DEBUG` variable in the container will be taken from the value for the same variable in the shell in which Compose is run.
+
+
+## The “env_file” configuration option
+
+You can pass multiple environment variables from an external file through to a service's containers with the ['env_file' option](compose-file.md#env-file), just like with `docker run --env-file=FILE ...`:
+
+    web:
+      env_file:
+        - web-variables.env
+
+
+## Setting environment variables with 'docker-compose run'
+
+Just like with `docker run -e`, you can set environment variables on a one-off container with `docker-compose run -e`:
+
+    $ docker-compose run -e DEBUG=1 web python console.py
+
+You can also pass a variable through from the shell by not giving it a value:
+
+    $ docker-compose run -e DEBUG web python console.py
+
+The value of the `DEBUG` variable in the container will be taken from the value for the same variable in the shell in which Compose is run.
+
+
+## The “.env” file
+
+You can set default values for any environment variables referenced in the Compose file, or used to configure Compose, in an [environment file](env-file.md) named `.env`:
+
+    $ cat .env
+    TAG=v1.5
+
+    $ cat docker-compose.yml
+    version: '2.0'
+    services:
+      web:
+        image: "webapp:${TAG}"
+
+When you run `docker-compose up`, the `web` service defined above uses the image `webapp:v1.5`. You can verify this with the [config command](reference/config.md), which prints your resolved application config to the terminal:
+
+    $ docker-compose config
+    version: '2.0'
+    services:
+      web:
+        image: 'webapp:v1.5'
+
+Values in the shell take precedence over those specified in the `.env` file. If you set `TAG` to a different value in your shell, the substitution in `image` uses that instead:
+
+    $ export TAG=v2.0
+
+    $ docker-compose config
+    version: '2.0'
+    services:
+      web:
+        image: 'webapp:v2.0'
+
+## Configuring Compose using environment variables
+
+Several environment variables are available for you to configure the Docker Compose command-line behaviour. They begin with `COMPOSE_` or `DOCKER_`, and are documented in [CLI Environment Variables](reference/envvars.md).
+
+
+## Environment variables created by links
+
+When using the ['links' option](compose-file.md#links) in a [v1 Compose file](compose-file.md#version-1), environment variables will be created for each link. They are documented in the [Link environment variables reference](link-env-deprecated.md). Please note, however, that these variables are deprecated - you should just use the link alias as a hostname instead.