Use gcc8 as default in CI container

yocto 2.6 uses gcc8 as it's default so make the CI
docker container use the same for compatibility

Resolves openbmc/openbmc-build-scripts#23

Change-Id: If55b17b6db8e82174016f7ff7c34533ca329285c
Signed-off-by: Andrew Geissler <geissonator@yahoo.com>
diff --git a/build-unit-test-docker.sh b/build-unit-test-docker.sh
index 0c4289f..bd02a1d 100755
--- a/build-unit-test-docker.sh
+++ b/build-unit-test-docker.sh
@@ -153,8 +153,8 @@
 RUN cat /etc/apt/sources.list.d/debug.list
 
 RUN apt-get update && apt-get install -yy \
-    gcc \
-    g++ \
+    gcc-8 \
+    g++-8 \
     libc6-dbg \
     libc6-dev \
     libtool \
@@ -209,6 +209,9 @@
     libprotobuf-dev \
     protobuf-compiler
 
+# Make gcc8 and g++8 the default
+RUN update-alternatives --install /usr/bin/gcc gcc /usr/bin/gcc-8 800 --slave /usr/bin/g++ g++ /usr/bin/g++-8
+
 RUN pip install inflection
 RUN pip install pycodestyle
 RUN pip3 install meson==0.49.0