build-unit-test-docker: Add dbus

Add dbus to the base image for dbus-1.pc.  This lets us not hardcode
things like system_bus_services_dir.  For example, consider this
meson.build fragment:

dbus_system_bus_services_dir = dependency('dbus-1').get_pkgconfig_variable(
    'system_bus_services_dir',
    define_variable: ['prefix', get_option('prefix')])

Change-Id: I003f41d23d5a68a8e3e0f8b5e409edc4515cf437
Signed-off-by: Brad Bishop <bradleyb@fuzziesquirrel.com>
diff --git a/build-unit-test-docker.sh b/build-unit-test-docker.sh
index 8422bf5..a3299c0 100755
--- a/build-unit-test-docker.sh
+++ b/build-unit-test-docker.sh
@@ -209,6 +209,7 @@
     libc6-dev \
     libtool \
     bison \
+    dbus \
     flex \
     cmake \
     python3 \