When building the Dockerfile, the Seafile GPG key import randomly fails.

Here's a log of the issue:

gpg: requesting key D401AB61 from hkp server keyserver.ubuntu.com
gpgkeys: key 8756C4F765C9AC3CB6B85D62379CE192D401AB61 can't be retrieved
gpg: no valid OpenPGP data found.
gpg: Total number processed: 0