Make backup report hostname configurable

lib/report.py

@@ -50,7 +50,8 @@ def human(value):
 
 
 def deliver(args):
-    host = socket.getfqdn()
+    fqdn = socket.getfqdn()
+    host = args.hostname or fqdn
     now = dt.datetime.now().astimezone()
     state = 'SUCCESS' if args.exit_code == 0 else 'FAIL'
     try:
@@ -89,7 +90,7 @@ def deliver(args):
     body.extend(['', f'Log-Auszug (letzte {args.tail_lines} Zeilen)', '-----------------', tail])
     message = EmailMessage(policy=SMTP)
     # EmailMessage rejects CR/LF in headers and encodes non-ASCII safely.
-    sender = args.sender or f'app-backup@{host}'
+    sender = args.sender or f'app-backup@{fqdn}'
     if not args.to.strip() or not sender.strip():
         raise ValueError('Mail recipient and sender must not be empty')
     message['From'] = sender
@@ -112,6 +113,7 @@ def main():
     for name in ('output', 'staging', 'archives'):
         snap.add_argument(name)
     mail = commands.add_parser('deliver')
+    mail.add_argument('--hostname', default='')
     for name in ('sizes', 'log', 'output', 'to', 'sender', 'subject-prefix', 'run', 'phase',
                  'upload-status', 'config', 'workdir', 'archive-dir', 'compression', 'remote', 'sendmail'):
         mail.add_argument('--'+name, required=True)