build-unit-test-docker: Fix googletest with c++17

If you don't compile with c++17 support at build time, then some of the
matchers like those of std::string_view don't get linked into gmock
correctly.

Change-Id: I8d89cfafa46037ef5829eecfbf3506d9fefcb6a1
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 f742083..8422bf5 100755
--- a/build-unit-test-docker.sh
+++ b/build-unit-test-docker.sh
@@ -297,7 +297,7 @@
 cd googletest-* && \
 mkdir build && \
 cd build && \
-cmake ${CMAKE_FLAGS[@]} -DTHREADS_PREFER_PTHREAD_FLAG=ON .. && \
+CXXFLAGS=-std=c++17 cmake ${CMAKE_FLAGS[@]} -DTHREADS_PREFER_PTHREAD_FLAG=ON .. && \
 make -j$(nproc) && \
 make install