docker: build libyang2 along with FRR

docker: build libyang2 along with FRR

Alpine images have been broken for some time because libyang2 is not
available in Alpine. This patch updates our Dockerfile to build a
libyang2 APK and install it into the image to satisfy FRR's libyang2
dependency.

Unfortunately, libyang2 erroneously includes an internal header from
glibc, making it dependent on glibc to build. FRR's official Docker
images are based on Alpine, which only offers musl libc. Until libyang2
fixes this problem, the libyang2 source that is installed in this image
is a patched version that is compatible with musl libc and not an
official version.

Signed-off-by: Quentin Young <qlyoung@nvidia.com>