docker: switch from kernel crosstools compilers to Debian cross-compi… …lers Now it is possible to build fully functional OpenBIOS binaries for all architectures using the Debian cross-compiler packages, so switch the builder image from the kernel crosstools compilers to the packaged Debian cross-compilers. Sigend-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>