build-unit-test-docker: Upgrade system before install

Sometimes, the docker image gets out of date enough that the packages we
are installing depend on newer versions of core libraries. Apt doesn't
update the dependencies as needed for install, so update the system
first.

Change-Id: Ie4cdde49c27459cc3e36076a3de44f402eaf4f16
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 25a3cfd..b48aaa3 100755
--- a/build-unit-test-docker.sh
+++ b/build-unit-test-docker.sh
@@ -182,7 +182,7 @@
 
 RUN cat /etc/apt/sources.list.d/debug.list
 
-RUN apt-get update && apt-get install -yy \
+RUN apt-get update && apt-get dist-upgrade -yy && apt-get install -yy \
     gcc-10 \
     g++-10 \
     libc6-dbg \