Enable figuring out number of cores when running on OS X

Signed-off-by: Steven Cooreman <steven.cooreman@silabs.com>

tests/scripts/docker_env.sh

@@ -60,12 +60,19 @@ else
     DOCKER="sudo docker"
 fi
 
+# Figure out the number of processors available
+if [ "$(uname)" == "Darwin" ]; then
+    NUM_PROC="$(sysctl -n hw.logicalcpu)"
+else
+    NUM_PROC="$(nproc)"
+fi
+
 # Build the Docker image
 echo "Getting docker image up to date (this may take a few minutes)..."
 ${DOCKER} image build \
     -t ${DOCKER_IMAGE_TAG} \
     --cache-from=${DOCKER_IMAGE_TAG} \
-    --build-arg MAKEFLAGS_PARALLEL="-j $(nproc)" \
+    --build-arg MAKEFLAGS_PARALLEL="-j ${NUM_PROC}" \
     --network host \
     ${http_proxy+--build-arg http_proxy=${http_proxy}} \
     ${https_proxy+--build-arg https_proxy=${https_proxy}} \