Clarify --env-file usage with names without values
@@ -393,17 +393,20 @@ If no `=` is provided and that variable is not exported in your local
environment, the variable won't be set in the container.
You can also load the environment variables from a file. This file should use
-the syntax `<variable>= value`, and `#` for comments.
+the syntax `<variable>=value` (which sets the variable to the given value) or
+`<variable>` (which takes the value from the local environment), and `#` for comments.
```bash
$ cat env.list
# This is a comment
VAR1=value1
VAR2=value2
+USER
$ docker run --env-file env.list ubuntu env | grep VAR
VAR1=value1
VAR2=value2
+USER=denis
```
### Set metadata on container (-l, --label, --label-file)
