Remove packages from first stage that are needed only in the second one

Dockerfile

 FROM debian:buster AS build
 RUN apt-get -q update && env DEBIAN_FRONTEND=noninteractive apt-get install -y --no-install-recommends \
-    build-essential pkg-config golang-go git ca-certificates openvpn netcat-openbsd iptables\
+    build-essential pkg-config golang-go git ca-certificates \
 && rm -rf /var/lib/apt/lists/*
 
 # don't need to do bash tricks to keep the layers small, as this is a multi-stage build