Dockerfile: switch to iptables-legacy to match the host

CI runs on Ubuntu 16.04 machines, which use iptables (legacy), but
Debian buster uses nftables. Because of this, DNS resolution does not
work if the daemon configures iptables.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>
(cherry picked from commit bb0472bd23806327a1ff1ea87343a6a0cadc21e9)
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>
Upstream-commit: 68db0c173965da830140fc273fb98feb00cda031
Component: engine

Author: thaJeztah

components/engine/Dockerfile

@@ -281,6 +281,11 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
         zip \
     && rm -rf /var/lib/apt/lists/*
 
+# Switch to use iptables instead of nftables (to match the host machine)
+RUN update-alternatives --set iptables  /usr/sbin/iptables-legacy  || true \
+ && update-alternatives --set ip6tables /usr/sbin/ip6tables-legacy || true \
+ && update-alternatives --set arptables /usr/sbin/arptables-legacy || true
+
 RUN pip3 install yamllint==1.16.0
 
 COPY --from=dockercli     /build/ /usr/local/cli