Thank you for the hint. Unfortunately, I always get this error message:
/usr/bin/docker: stderr failed to copy: read tcp 10.20.0.12:33562->80.228.47.67:443: read: connection reset by peer
Error: Failed command: /usr/bin/docker pull quay.io/ceph/ceph:v19
On the other hand, the host in question does react to a ping.
Update: I found out the cause of the break in connection: missing proxy settings for Docker!