container: Script fails due to gpg keyserver error

$ ./container up --distribution=stable
> Receiving GPG keys
gpg: keyserver receive failed: No name
Traceback (most recent call last):
  File "/home/james/freedombox/freedombox/./container", line 1026, in <module>
    main()
  File "/home/james/freedombox/freedombox/./container", line 1022, in main
    subcommand_method(arguments)
  File "/home/james/freedombox/freedombox/./container", line 952, in subcommand_up
    image_file = _download_disk_image(arguments.distribution)
  File "/home/james/freedombox/freedombox/./container", line 504, in _download_disk_image
    _verify_signature(target_file, signature_file)
  File "/home/james/freedombox/freedombox/./container", line 426, in _verify_signature
    subprocess.run([
  File "/usr/lib/python3.9/subprocess.py", line 528, in run
    raise CalledProcessError(retcode, process.args,
subprocess.CalledProcessError: Command '['gpg', '--quiet', '--homedir', '/home/james/freedombox/freedombox/.container/gpg', '--keyserver', 'keys.gnupg.net', '--recv-keys', '013D86D8BA32EAB4A6691BF85D4153D6FE188FC8']' returned non-zero exit status 2.

I already have the key in this keyring, so maybe we can skip trying to fetch the key in this case.