Makefile,Dockerfile: add multi-arch images

This commit changes the build-system for Kilo to create container images
for multiple architectures. This will enable running Kilo on Arm
devices, e.g. Raspberry Pis. This is accomplished using Docker
manifests.

Dockerfile

@@ -1,12 +1,14 @@
+ARG FROM=alpine
 FROM alpine AS cni
 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 && \
     tar -xf cni.tar.gz
 
-FROM alpine
-MAINTAINER squat <lserven@gmail.com>
+FROM $FROM
+LABEL maintainer="squat <lserven@gmail.com>"
 RUN echo "@testing http://nl.alpinelinux.org/alpine/edge/testing" >> /etc/apk/repositories && \
     apk add --no-cache ipset iptables wireguard-tools@testing
 COPY --from=cni bridge host-local loopback portmap /opt/cni/bin/
-COPY bin/kg /opt/bin/
+ARG GOARCH
+COPY bin/$GOARCH/kg /opt/bin/
 ENTRYPOINT ["/opt/bin/kg"]