Attempt to document escaping env vars

People are likely to run into their env vars being set to empty strings,
if they're not aware that they need to escape them for Compose to not
interpolate them.

Signed-off-by: Mazz Mosley <[email protected]>

docs/compose-file.md

@@ -428,9 +428,18 @@ 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 (`$$`).
+You can use a `$$` (double-dollar sign) when your configuration needs a literal
+dollar sign. This also prevents Compose from interpolating a value, so a `$$`
+allows you to refer to environment variables that you don't want processed by
+Compose.
 
+    web:
+      build: .
+      command: "$$VAR_NOT_INTERPOLATED_BY_COMPOSE"
+
+If you forget and use a single dollar sign (`$`), Compose interprets the value as an environment variable and will warn you:
+
+  The VAR_NOT_INTERPOLATED_BY_COMPOSE is not set. Substituting an empty string.
 
 ## Compose documentation