"Device or resource busy" when i try move /etc/resolv.conf in ubuntu:18.04. How fix it?

I have a VPN client in my Docker container (ubuntu:18.04).

The client must do the following:

mv /etc/resolv.conf /etc/resolv.conf.orig

Then the client should create new /etc/resolv.conf with their DNS servers. However, the move fails with an error:

mv: cannot move '/etc/resolv.conf' to '/etc/resolv.conf.orig': Device or resource busy

Can this be fixed?

P.S.: I can 't change the VPN client code.