make buildx: use multi-stage to make tagged image smaller

The build-stage would still be in the local cache (and can
be cleaned up with `docker system prune`) but the tagged
image (which usually wouldn't be removed) will take up
less space now.

Note that

 - the binary is not statically linked, so we cannot use
   a "from scratch" image
 - in cases where the binary is cross-compiled (e.g.
   on a non-linux machine), the image itself is not
   really useful (we may want to consider not tagging
   the image in that situation)

Before:

    REPOSITORY      TAG     IMAGE ID      CREATED         SIZE
    moby-buildx     latest  c9b2af465baf  7 minutes ago   1.71GB

After:

    REPOSITORY      TAG     IMAGE ID      CREATED         SIZE
    moby-buildx     latest  345501e2df0a  2 minutes ago   820MB

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

Dockerfile.buildx

@@ -2,7 +2,7 @@ ARG GO_VERSION=1.12.10
 ARG BUILDX_COMMIT=v0.3.0
 ARG BUILDX_REPO=https://github.com/docker/buildx.git
 
-FROM golang:${GO_VERSION}-stretch
+FROM golang:${GO_VERSION}-stretch AS build
 ARG BUILDX_REPO
 RUN git clone "${BUILDX_REPO}" /buildx
 WORKDIR /buildx
@@ -21,4 +21,7 @@ RUN GOOS="${GOOS}" GOARCH="${GOARCH}" BUILDX_COMMIT="${BUILDX_COMMIT}"; \
       -X \"${pkg}/version.Package=buildx\" \
     "; \
     go build -ldflags "${ldflags}" -o /usr/bin/buildx ./cmd/buildx
+
+FROM golang:${GO_VERSION}-stretch
+COPY --from=build /usr/bin/buildx /usr/bin/buildx
 ENTRYPOINT ["/usr/bin/buildx"]