Mailcow: Hardcode Nginx image version.

Change-Id: I6142b91fac84d1cb59ef1a247e29aac90580d73c
diff --git a/mailcow/src/mailcow-dockerized/docker-compose.yml b/mailcow/src/mailcow-dockerized/docker-compose.yml
index a1cb21a..2ec3fa0 100644
--- a/mailcow/src/mailcow-dockerized/docker-compose.yml
+++ b/mailcow/src/mailcow-dockerized/docker-compose.yml
@@ -304,7 +304,7 @@
         - sogo-mailcow
         - php-fpm-mailcow
         - redis-mailcow
-      image: nginx:mainline-alpine
+      image: nginx:1.19-alpine
       dns:
         - ${IPV4_NETWORK:-172.22.1}.254
       command: /bin/sh -c "envsubst < /etc/nginx/conf.d/templates/listen_plain.template > /etc/nginx/conf.d/listen_plain.active &&