build-unit-test-docker: Consolidate make invocations

Consistency fix

Change-Id: I8f27123010eb139b2e68267623d95f22faa2d3a1
Signed-off-by: William A. Kennington III <wak@google.com>
diff --git a/build-unit-test-docker.sh b/build-unit-test-docker.sh
index dbbc320..0b25a52 100755
--- a/build-unit-test-docker.sh
+++ b/build-unit-test-docker.sh
@@ -180,7 +180,8 @@
 mkdir build && \
 cd build && \
 cmake ${CMAKE_FLAGS[@]} -DBUILD_GTEST=ON -DBUILD_GMOCK=ON .. && \
-make && make install
+make -j$(nproc) && \
+make install
 
 RUN curl -L https://github.com/USCiLab/cereal/archive/v1.2.2.tar.gz | tar -xz && \
 cp -a cereal-1.2.2/include/cereal/ ${PREFIX}/include/