fix: Write ENABLE_MTA_STS to /etc/dms-settings (docker-mailserver#4707)

Author: polarathene

target/scripts/startup/variables-stack.sh

@@ -100,6 +100,7 @@ function __environment_variables_general_setup() {
   VARS[ENABLE_FETCHMAIL]="${ENABLE_FETCHMAIL:=0}"
   VARS[ENABLE_GETMAIL]="${ENABLE_GETMAIL:=0}"
   VARS[ENABLE_MANAGESIEVE]="${ENABLE_MANAGESIEVE:=0}"
+  VARS[ENABLE_MTA_STS]="${ENABLE_MTA_STS:=0}"
   VARS[ENABLE_OAUTH2]="${ENABLE_OAUTH2:=0}"
   VARS[ENABLE_OPENDKIM]="${ENABLE_OPENDKIM:=1}"
   VARS[ENABLE_OPENDMARC]="${ENABLE_OPENDMARC:=1}"