fix minor convention issues across 10 stacks

mailu_dev/compose.yaml

@@ -1,6 +1,6 @@
 services:
   redis:
-    image: redis:alpine
+    image: ${MAILU_REDIS_IMAGE:-redis:alpine}
     restart: ${MAILU_RESTART:-always}
     volumes:
       - ${VOL_PATH:-/data}/mailu/redis:/data

mailu_dev/sample.env

@@ -5,6 +5,7 @@ MAILU_VERSION=2024.06
 MAILU_RESTART=always
 MAILU_ORG=ghcr.io/mailu
 MAILU_WEBMAIL=roundcube
+MAILU_REDIS_IMAGE=redis:alpine
 
 # Domain
 DOMAIN=example.com