Update xenial.docker

dockerfiles/xenial.docker

@@ -7,6 +7,7 @@ RUN apt-get update && \
     apt-get install -y software-properties-common apt-utils && \
     add-apt-repository -y universe && \
     add-apt-repository -y ppa:hola-launchpad/valgrind && \
+    add-apt-repository -y ppa:git-core/ppa && \
     apt-get update && \
     apt-get upgrade -y && \
     apt-get dist-upgrade -y && \