Dockerfile: use TARGETPLATFORM to build Docker
follow-up #43529
~needs https://github.com/moby/moby/pull/44513~

fixes https://github.com/moby/moby/pull/41590
fixes https://github.com/moby/moby/issues/41862
fixes https://github.com/moby/moby/issues/41519
fixes https://github.com/moby/moby/issues/39380
closes https://github.com/moby/moby/pull/43492
closes https://github.com/moby/moby/pull/43613

Better support for cross compilation so we can fully rely on `--platform` flag of buildx for a seamless integration.

This removes unnecessary extra cross logic in the Dockerfile, removes `DOCKER_CROSSPLATFORMS` and `CROSS` vars and some hack scripts as well.

Non-sandboxed build invocation is still supported and dev stages in the Dockerfile have been updated accordingly.

Bake definition and GitHub Actions workflows have been updated accordingly as well.

cc @thaJeztah @rumpl @vvoland 
dockerversion/version_lib.go
package dockerversion // import "github.com/docker/docker/dockerversion"

// Default build-time variable for library-import.
// These variables are overridden on build with build-time information.
var (
	GitCommit             = "library-import"
	Version               = "library-import"
	BuildTime             = "library-import"
	IAmStatic             = "library-import"
	PlatformName          = ""
	ProductName           = ""
	DefaultProductLicense = ""
)

package dockerversion // import "github.com/docker/docker/dockerversion"

// Default build-time variable for library-import.
// These variables are overridden on build with build-time information.
var (
	GitCommit             = "library-import"
	Version               = "library-import"
	BuildTime             = "library-import"
	PlatformName          = ""
	ProductName           = ""
	DefaultProductLicense = ""
)

ISTR we kept this field in `dockerversion` with the intent to (potentially) surface this information in `docker version` / `docker info` and/or `--version` output to help debugging issues when users report issues (it would save us to ask "is this a static binary, or did you install from deb/rpm?").

I don't think we ever did this, but perhaps still something to consider.