Add build args

args: operating system (OS) and architecture (ARCH)

Dockerfile

@@ -1,5 +1,8 @@
 FROM golang:alpine as builder
 
+ARG OS
+ARG ARCH
+
 WORKDIR /go/src/github.com/eko/pihole-exporter
 COPY . .
 
@@ -7,7 +10,7 @@ RUN apk update && \
     apk --no-cache add git alpine-sdk upx
 
 RUN GO111MODULE=on go mod vendor
-RUN CGO_ENABLED=0 GOOS=linux GOARCH=amd64 go build -ldflags '-s -w' -o binary ./
+RUN CGO_ENABLED=0 GOOS=$OS GOARCH=$ARCH go build -ldflags '-s -w' -o binary ./
 RUN upx -f --brute binary
 
 FROM scratch