Speed up crossplatform Docker image building

Dockerfile

@@ -1,13 +1,20 @@
 # docker build -t msoap/shell2http .
 
 # build image
-FROM golang:alpine as go_builder
+FROM --platform=$BUILDPLATFORM golang:alpine as go_builder
 
 RUN apk add --no-cache git
 
 ADD . $GOPATH/src/github.com/msoap/shell2http
 WORKDIR $GOPATH/src/github.com/msoap/shell2http
+
 ENV CGO_ENABLED=0
+# GOARM=6 affects only "arm" builds
+ENV GOARM=6
+# "amd64", "arm64" or "arm" (--platform=linux/amd64,linux/arm64,linux/arm/v6)
+ENV GOARCH=$TARGETARCH
+ENV GOOS=linux
+
 RUN go build -v -trimpath -ldflags="-w -s -X 'main.version=$(git describe --abbrev=0 --tags | sed s/v//)'" -o /go/bin/shell2http .
 
 # final image