Use all Docker CPU:s by default for CMake

Use e.g. --cpuset-cpus or --cpu-quota to externally limit CPU usage with
docker.  The analogous make -j may not work for the Makefile build,
because make is run recursively.

Author: hattne

.dev/docker/cmake_ctest.dockerfile

@@ -9,7 +9,7 @@ RUN apt-get update && \
 
 RUN cd /app/cbflib && \
   cmake . && \
-  cmake --build . --parallel 4
+  cmake --build . --parallel `nproc`
 
 RUN cd /app/cbflib && \
-  ctest --parallel 4
+  ctest --parallel `nproc`