Documentation for .env file

Signed-off-by: Joffrey F <joffrey@docker.com>

docs/env-file.md

@@ -0,0 +1,38 @@
+<!--[metadata]>
++++
+title = "Environment file"
+description = "Declaring default environment variables in file"
+keywords = ["fig, composition, compose, docker, orchestration, environment, env file"]
+[menu.main]
+parent = "workw_compose"
+weight=10
++++
+<![end-metadata]-->
+
+
+# Environment file
+
+Compose supports declaring default environment variables in an environment
+file named `.env` and placed in the same folder as your
+[compose file](compose-file.md).
+
+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.
+
+> Note: Values present in the environment at runtime will always override
+> those defined inside the `.env` file.
+
+Those environment variables will be used for
+[variable substitution](compose-file.md#variable-substitution) in your Compose
+file, but can also be used to define the following
+[CLI variables](reference/envvars.md):
+
+- `COMPOSE_PROJECT_NAME`
+- `COMPOSE_FILE`
+- `COMPOSE_API_VERSION`
+
+## More Compose documentation
+
+- [User guide](index.md)
+- [Command line reference](./reference/index.md)
+- [Compose file reference](compose-file.md)

docs/index.md

@@ -23,6 +23,7 @@ Compose is a tool for defining and running multi-container Docker applications.
 - [Frequently asked questions](faq.md)
 - [Command line reference](./reference/index.md)
 - [Compose file reference](compose-file.md)
+- [Environment file](env-file.md)
 
 To see a detailed list of changes for past and current releases of Docker
 Compose, please refer to the