The value of the capability argument is not permitted for a file. Or the file is

not a regular (non-symlink) file

/sbin/ip is a symlink to /bin/ip

Dockerfile

@@ -20,5 +20,5 @@ RUN apt-get -q update && env DEBIAN_FRONTEND=noninteractive apt-get install -y -
         openvpn libcap2-bin netcat-openbsd iptables\
         && rm -rf /var/lib/apt/lists/*
 RUN setcap cap_net_admin,cap_net_bind_service+ep /usr/sbin/openvpn
-RUN setcap cap_net_admin+ep /sbin/ip
+RUN setcap cap_net_admin+ep /bin/ip