docker: introduce Dockerfile.builder for fcode-utils-builder container

This introduces a new Dockerfile.builder file that can be used by docker
build to generate a container that can build fcode-utils based upon Debian
11.2.

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

Author: mcayland

docker/Dockerfile.builder

@@ -0,0 +1,4 @@
+FROM debian:11.2
+
+RUN apt-get -y update && \
+    apt-get -y install build-essential git