i start the container and everything works. client can connect.
stop the container and start it again.
directly after the start the folder /etc/openvpn/easy-rsa is deleted.
while all the keys and certificates are in there.
mounting the folder /etc/openvpn outside the container in a volume does not help.
i don't want to wait 40 mins every start nor copy the client.ovpn file to the client every start of the container