Add exit status to docker exec manpage

There's little way of knowing what each exit status means at present
because it's not documented. I'm assuming they are the same as docker
run.

Signed-off-by: Eric Curtin <ericcurtin17@gmail.com>

man/src/container/exec.md

@@ -23,3 +23,29 @@ the same capabilities as the container, which may be limited. Set
    --user [user | user:group | uid | uid:gid | user:gid | uid:group ]
 
    Without this argument the command will be run as root in the container.
+
+# Exit Status
+
+The exit code from `docker exec` gives information about why the container
+failed to exec or why it exited.  When `docker exec` exits with a non-zero code,
+the exit codes follow the `chroot` standard, see below:
+
+**_126_** if the **_contained command_** cannot be invoked
+
+    $ docker exec busybox /etc; echo $?
+    # exec: "/etc": permission denied
+      docker: Error response from daemon: Contained command could not be invoked
+      126
+
+**_127_** if the **_contained command_** cannot be found
+
+    $ docker exec busybox foo; echo $?
+    # exec: "foo": executable file not found in $PATH
+      docker: Error response from daemon: Contained command not found or does not exist
+      127
+
+**_Exit code_** of **_contained command_** otherwise 
+    
+    $ docker exec busybox /bin/sh -c 'exit 3' 
+    # 3
+