feat(tests): update Dockerfile and CI script for parallel builds and improved caching

Dockerfile.debian

@@ -3,11 +3,13 @@ FROM debian:testing
 
 ENV DEBIAN_FRONTEND="noninteractive"
 ENV TERM=xterm
+ENV CC="ccache gcc"
 
 RUN apt-get update && apt-get install -y --no-install-recommends \
   autoconf \
   autoconf-archive \
   automake \
+  ccache \
   gcc \
   git \
   libcmocka-dev \