build-unit-test-docker: Install python3-git

Required by dbus-unit-test.

Change-Id: Ief51df90d8fd5e2276a96e7d129c7db36dae6436
Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
diff --git a/build-unit-test-docker.sh b/build-unit-test-docker.sh
index c649971..65708f4 100755
--- a/build-unit-test-docker.sh
+++ b/build-unit-test-docker.sh
@@ -201,6 +201,7 @@
     python3-mako \
     python3-pip \
     python3-setuptools \
+    python3-git \
     pkg-config \
     autoconf \
     autoconf-archive \