Update env.example to reflect latest Mattermost version.

env.example

@@ -55,7 +55,7 @@ MATTERMOST_CLIENT_PLUGINS_PATH=./volumes/app/mattermost/client/plugins
 
 ## This will be 'mattermost-enterprise-edition' or 'mattermost-team-edition' based on the version of Mattermost you're installing.
 MATTERMOST_IMAGE=mattermost-enterprise-edition
-MATTERMOST_IMAGE_TAG=5.37
+MATTERMOST_IMAGE_TAG=5.38
 
 ## The app port is only relevant for using Mattermost without the nginx container as reverse proxy. This is not meant
 ## to be used with the internal HTTP server exposed but rather in case one wants to host several services on one host