docker: introduce Dockerfile.builder for openbios-builder container

This introduces a new Dockerfile.builder file that can be used by docker
build to generate a container that can build OpenBIOS based upon Debian
11.2 and the crosstool compilers from kernel.org.

The installed cross-compilers allow the building of SPARC32, SPARC64 and
PPC binaries.

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>