limit default parallel processes for contrib/docker/Makefile (#582)

* limited parallel make processes to make -j 16 by default for contrib/docker/Makefile

* set the default to make -j 22 for parallel make in contrib/docker/Makefile

contrib/docker/Makefile

 # Basic Makefile for contrib/docker. This can be expanded later as more targets
 # are added.
 
-# Default is to build with -j for parallel cmake/make.  Turn off with
-#   make PARALLEL=
-PARALLEL=-j
+# Building LLVM from source has been observed to trigger the oom-killer
+#    on systems with a large number of cores
+#    running with make -j
+#
+# Default is to build with -j 22 for parallel cmake/make.
+# Override with make PARALLEL="-j <num_parallel_processes>" where
+#    <num_parallel_processes> = the number of make processes to run in parallel
+# Turn off with make PARALLEL=
+PARALLEL=-j 22 
 
 # DIR is an internal variable that serves as an anchor to this cloned git
 # repository.  DIR is mounted into the docker container, so that builds