Is it possible to (manually) build multi-arch docker image without docker buildx?

Question

I have a Golang project which is built and put into scratch image:

# Build binary
FROM golang:1.17-alpine AS build-env
ADD . /app
WORKDIR /app
RUN env CGO_ENABLED=0 go build -ldflags="-s -w" -o myapp ./cmd/myapp/main.go

# Create image
FROM scratch
COPY --from=build-env /app/myapp /
ENTRYPOINT ["/myapp"]

Since Golang allows to cross-compile for multiple architectures (and it's way faster than building in emulated environments), I want to do something like this:

# Build binary
FROM golang:1.17-alpine AS build-env
ADD . /app
WORKDIR /app
RUN env CGO_ENABLED=0 GOOS=linux GOARCH=386 go build -ldflags="-s -w" -o myapp_linux_i386 ./cmd/myapp/main.go             # Linux 32bit
RUN env CGO_ENABLED=0 GOOS=linux GOARCH=amd64 go build -ldflags="-s -w" -o myapp_linux_x86_64 ./cmd/myapp/main.go         # Linux 64bit
RUN env CGO_ENABLED=0 GOOS=linux GOARCH=arm GOARM=5 go build -ldflags="-s -w" -o myapp_linux_arm ./cmd/myapp/main.go      # Linux armv5/armel/arm (it also works on armv6)
RUN env CGO_ENABLED=0 GOOS=linux GOARCH=arm GOARM=7 go build -ldflags="-s -w" -o myapp_linux_armhf ./cmd/myapp/main.go    # Linux armv7/armhf
RUN env CGO_ENABLED=0 GOOS=linux GOARCH=arm64 go build -ldflags="-s -w" -o myapp_linux_aarch64 ./cmd/myapp/main.go        # Linux armv8/aarch64
RUN env CGO_ENABLED=0 GOOS=freebsd GOARCH=amd64 go build -ldflags="-s -w" -o myapp_freebsd_x86_64 ./cmd/myapp/main.go     # FreeBSD 64bit
RUN env CGO_ENABLED=0 GOOS=darwin GOARCH=amd64 go build -ldflags="-s -w" -o myapp_darwin_x86_64 ./cmd/myapp/main.go       # Darwin 64bit
RUN env CGO_ENABLED=0 GOOS=darwin GOARCH=arm64 go build -ldflags="-s -w" -o myapp_darwin_aarch64 ./cmd/myapp/main.go      # Darwin armv8/aarch64
RUN env CGO_ENABLED=0 GOOS=windows GOARCH=386 go build -ldflags="-s -w" -o myapp_windows_i386.exe ./cmd/myapp/main.go     # Windows 32bit
RUN env CGO_ENABLED=0 GOOS=windows GOARCH=amd64 go build -ldflags="-s -w" -o myapp_windows_x86_64.exe ./cmd/myapp/main.go # Windows 64bit

But then how do I create and push multi-arch docker images?

# Create image
...