Added usage example for -e for exec

Signed-off-by: Vishnu Narayanan <iamwishnu@gmail.com>
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>
(cherry picked from commit c848d9b22d)
Signed-off-by: Andrew Hsu <andrewhsu@docker.com>

components/cli/docs/reference/commandline/exec.md

@@ -76,6 +76,17 @@ $ docker exec -it ubuntu_bash bash
 
 This will create a new Bash session in the container `ubuntu_bash`.
 
+Next, set an environment variable in the current bash session.
+
+```bash
+$ docker exec -it -e VAR=1 ubuntu_bash bash
+```
+
+This will create a new Bash session in the container `ubuntu_bash` with environment 
+variable `$VAR` set to "1". Note that this environment variable will only be valid 
+on the current Bash session.
+
+
 ### Try to run `docker exec` on a paused container
 
 If the container is paused, then the `docker exec` command will fail with an error: