Update Dockerfile for cross-compilation

Dockerfile

@@ -1,17 +1,17 @@
 # Step 1: Modules caching
-FROM golang:latest as modules
+FROM --platform=$BUILDPLATFORM golang:latest as modules
 COPY go.mod go.sum /modules/
 WORKDIR /modules
 RUN go mod download
 
-# Step 2: Builder
-FROM golang:latest AS builder
+# Step 2: Builder, with cross compile
+FROM --platform=$BUILDPLATFORM golang:latest AS builder
 COPY --from=modules /go/pkg /go/pkg
 COPY . /app
-ARG TARGETPLATFORM
+ARG TARGETOS TARGETARCH 
 ENV CGO_ENABLED=0
 WORKDIR /app
-RUN go build -o /bin/app .
+RUN GOOS=$TARGETOS GOARCH=$TARGETARCH go build -o /bin/app .
 
 # GOPATH for scratch images is /
 FROM scratch