Move common flag descriptions to the buildx build reference, and make
that page the canonical page in docs. Also rewrite some content in
image_build to make clear that this page is only for the legacy builder.
Signed-off-by: David Karlsson <35727626+dvdksn@users.noreply.github.com>
(cherry picked from commit e91f0ded9c)
Signed-off-by: David Karlsson <35727626+dvdksn@users.noreply.github.com>
David Karlsson