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 
hack/make/dynbinary-proxy
#!/usr/bin/env bash

set -e

(
	export IAMSTATIC='false'
	export LDFLAGS_STATIC_DOCKER=''
	export BUILDFLAGS=("${BUILDFLAGS[@]/netgo /}")        # disable netgo, since we don't need it for a dynamic binary
	export BUILDFLAGS=("${BUILDFLAGS[@]/osusergo /}")     # ditto for osusergo
	export BUILDFLAGS=("${BUILDFLAGS[@]/static_build /}") # we're not building a "static" binary here

	GO_PACKAGE='github.com/docker/docker/cmd/docker-proxy'
	BINARY_NAME='docker-proxy'
	source "${MAKEDIR}/.binary"
)

#!/usr/bin/env bash

set -e

(
	export LDFLAGS_STATIC_DOCKER=''
	export BUILDFLAGS=("${BUILDFLAGS[@]/netgo /}")        # disable netgo, since we don't need it for a dynamic binary
	export BUILDFLAGS=("${BUILDFLAGS[@]/osusergo /}")     # ditto for osusergo
	export BUILDFLAGS=("${BUILDFLAGS[@]/static_build /}") # we're not building a "static" binary here

	GO_PACKAGE='github.com/docker/docker/cmd/docker-proxy'
	BINARY_NAME='docker-proxy'
	source "${MAKEDIR}/.binary"
)

Guess this is redundant, or do we explicitly check for `if DOCKER_STATIC = 0` anywhere?