Dockerfile: fix cni plugins for arm

This commit ensures that the architecture of the installed CNI plugins
corresponds to the architecture of the container.

Dockerfile

@@ -1,14 +1,15 @@
 ARG FROM=alpine
 FROM alpine AS cni
+ARG GOARCH
 RUN apk add --no-cache curl && \
-    curl -Lo cni.tar.gz https://github.com/containernetworking/plugins/releases/download/v0.7.5/cni-plugins-amd64-v0.7.5.tgz && \
+    curl -Lo cni.tar.gz https://github.com/containernetworking/plugins/releases/download/v0.7.5/cni-plugins-$GOARCH-v0.7.5.tgz && \
     tar -xf cni.tar.gz
 
 FROM $FROM
+ARG GOARCH
 LABEL maintainer="squat <lserven@gmail.com>"
 RUN echo "@community http://nl.alpinelinux.org/alpine/edge/community" >> /etc/apk/repositories && \
     apk add --no-cache ipset iptables wireguard-tools@community
 COPY --from=cni bridge host-local loopback portmap /opt/cni/bin/
-ARG GOARCH
 COPY bin/$GOARCH/kg /opt/bin/
 ENTRYPOINT ["/opt/bin/kg"]