docker: rework image pruning

with our new retagging approach, the Digest ID remains <null> because
this is only set by docker if truly fetched from the registry.

this means that redis container always gets removed...

src/mailserver.js

@@ -213,6 +213,8 @@ async function start(existingInfra) {
 
     debug('startMail: starting');
     await restart();
+
+    if (existingInfra.version !== 'none' && existingInfra.images.mail !== infra.images.mail) await docker.deleteImage(existingInfra.images.mail);
 }
 
 async function restartIfActivated() {