Mailcow: Use a persistent tag rather than 'latest'.

Change-Id: I7d60f45efa338c20056c16bb118d59adb66d4e47
diff --git a/mailcow/default.nix b/mailcow/default.nix
index 2691722..05a0b1d 100644
--- a/mailcow/default.nix
+++ b/mailcow/default.nix
@@ -99,7 +99,7 @@
 in
 img {
   name = "docker.benkard.de/mulk/mailcow";
-  tag = "latest";
+  #tag = "latest";
   maxLayers = 125;
   contents = extraDeps ++ maintenanceDeps;
   extraCommands =