Added usage example for -e for exec
@@ -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:
