Docker multi-arch images

Dockerfile

@@ -1,12 +1,23 @@
-FROM golang:1.15
+FROM --platform=$BUILDPLATFORM golang:1.15 AS build
+
+ARG TARGETOS
+ARG TARGETARCH
+ARG TARGETVARIANT
+
 RUN git clone https://github.com/chr-fritz/fritzbox_exporter.git /go/src/github.com/chr-fritz/fritzbox_exporter
 WORKDIR /go/src/github.com/chr-fritz/fritzbox_exporter
-RUN go mod download && \
-    CGO_ENABLED=0 GOOS=linux GOARCH=amd64 go build -a -ldflags '-w -extldflags "-static"' -o fritzbox-exporter .
 
-FROM alpine:latest
+RUN go mod download; \
+	case "$TARGETVARIANT" in \
+		v5) export GOARM=5 ;; \
+		v6) export GOARM=6 ;; \
+		v7) export GOARM=7 ;; \
+	esac; \
+	CGO_ENABLED=0 GOOS=$TARGETOS GOARCH=$TARGETARCH go build -a -ldflags '-w -extldflags "-static"' -o fritzbox-exporter .
+ 
+FROM --platform=$TARGETPLATFORM alpine:latest
 RUN apk --no-cache add ca-certificates
-COPY --from=0 /go/src/github.com/chr-fritz/fritzbox_exporter/fritzbox-exporter /fritzbox-exporter/
-COPY --from=0 /go/src/github.com/chr-fritz/fritzbox_exporter/*.json /etc/fritzbox-exporter/
+COPY --from=build /go/src/github.com/chr-fritz/fritzbox_exporter/fritzbox-exporter /fritzbox-exporter/
+COPY --from=build /go/src/github.com/chr-fritz/fritzbox_exporter/*.json /etc/fritzbox-exporter/
 ENTRYPOINT ["/fritzbox-exporter/fritzbox-exporter"]
 CMD ["--metrics-file","/etc/fritzbox-exporter/metrics.json","--listen-address","0.0.0.0:8080"]