Add multi-arch Dockerfile

Signed-off-by: Stefan Prodan <stefan.prodan@gmail.com>

Dockerfile

@@ -1,9 +1,29 @@
+FROM golang:1.15-alpine as builder
+
+ARG TARGETPLATFORM
+
+WORKDIR /workspace
+
+# copy modules manifests
+COPY go.mod go.mod
+COPY go.sum go.sum
+
+# cache modules
+RUN go mod download
+
+# copy source code
+COPY cmd/ cmd/
+COPY pkg/ pkg/
+
+# build
+RUN CGO_ENABLED=0 go build -a -o flagger ./cmd/flagger
+
 FROM alpine:3.12
 
 RUN apk --no-cache add ca-certificates
 
 USER nobody
 
-COPY --chown=nobody:nobody /bin/flagger .
+COPY --from=builder --chown=nobody:nobody /workspace/flagger .
 
 ENTRYPOINT ["./flagger"]