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/sftp.js

@@ -125,6 +125,8 @@ async function start(existingInfra) {
 
     debug('startSftp: starting sftp container');
     await shell.bash(runCmd, {});
+
+    if (existingInfra.version !== 'none' && existingInfra.images.sftp !== image) await docker.deleteImage(existingInfra.images.sftp);
 }
 
 async function status() {