docker: fix image prune

it seems docker images --digests cloudron/sftp --format "{{.ID}} {{.Repository}}:{{.Tag}}@{{.Digest}}
broke at some point

src/mailserver.js

@@ -141,7 +141,7 @@ async function configureMail(mailFqdn, mailDomain, serviceConfig) {
     // MAIL_DOMAIN is the domain for which this server is relaying mails
     // mail container uses /app/data for backed up data and /run for restart-able data
 
-    const tag = infra.images.mail.tag;
+    const image = infra.images.mail;
     const memoryLimit = serviceConfig.memoryLimit || exports.DEFAULT_MEMORY_LIMIT;
     const memory = await system.getMemoryAllocation(memoryLimit);
     const cloudronToken = hat(8 * 128), relayToken = hat(8 * 128);
@@ -187,7 +187,7 @@ async function configureMail(mailFqdn, mailDomain, serviceConfig) {
                 -v "${paths.MAIL_CONFIG_DIR}:/etc/mail:ro" \
                 ${ports} \
                 --label isCloudronManaged=true \
-                ${readOnly} -v /run -v /tmp ${tag} ${cmd}`;
+                ${readOnly} -v /run -v /tmp ${image} ${cmd}`;
 
     await shell.promises.exec('startMail', runCmd);
 }