docker: Build grub for all architectures

docker: Build grub for all architectures

For consistency now, and future ease of testing with non-amd64 hosts,
build grub for all architectures rather than relying on host binaries
for i386/x86_64.

Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Tom Rini <trini@konsulko.com>