Remove explicit OS and Arch builds

This could allow multi-arch supporting using `docker buildx`

Dockerfile

@@ -12,7 +12,7 @@ WORKDIR /go/src/github.com/dutchcoders/transfer.sh
 ENV GO111MODULE=on
 
 # build & install server
-RUN go get -u ./... && CGO_ENABLED=0 GOOS=linux GOARCH=amd64 go build -ldflags -a -tags netgo -ldflags '-w -extldflags "-static"' -o /go/bin/transfersh github.com/dutchcoders/transfer.sh
+RUN go get -u ./... && CGO_ENABLED=0 go build -ldflags -a -tags netgo -ldflags '-w -extldflags "-static"' -o /go/bin/transfersh github.com/dutchcoders/transfer.sh
 
 FROM scratch AS final
 LABEL maintainer="Andrea Spacca <andrea.spacca@gmail.com>"