build: docker: add protobuf libraries

Add installation of protobuf libraries to CI unit-test docker script to
enable packages with this dependency.

Change-Id: I4cd1e3b0aff1b22f7cf062c16c0a17fd948ea78a
Signed-off-by: Patrick Venture <venture@google.com>
diff --git a/build-unit-test-docker.sh b/build-unit-test-docker.sh
index c6fabde..24745da 100755
--- a/build-unit-test-docker.sh
+++ b/build-unit-test-docker.sh
@@ -194,7 +194,9 @@
     xxd \
     libi2c-dev \
     wget \
-    libldap2-dev
+    libldap2-dev \
+    libprotobuf-dev \
+    protobuf-compiler
 
 RUN pip install inflection
 RUN pip install pycodestyle