fix: Map MESSAGE_PLATFORM env var for monocker Discord notifications

The env_file directive passes MONOCKER_MESSAGE_PLATFORM verbatim, but
the container expects MESSAGE_PLATFORM. Added explicit mapping in the
environment section.

Author: ilude

services-available/monocker.yml

@@ -17,7 +17,7 @@ services:
       TZ: ${TZ}
       # Optional label to preface messages. Handy if you are running multiple versions of Monocker
       SERVER_LABEL: ${MONOCKER_SERVER_LABEL:-${HOST_NAME}}
-      # MESSAGE_PLATFORM is set via monocker.env file (loaded by env_file directive)
+      MESSAGE_PLATFORM: ${MONOCKER_MESSAGE_PLATFORM:-}
       # Optional - includes or excludes specified containers - default behaviour is false
       LABEL_ENABLE: ${MONOCKER_LABEL_ENABLE:-false}
       # Optional - only show when container state changes to being offline (paused, exited, running (unhealthy), or dead) - default is false