Merge branch 'ashmckenzie/7516-update-qa-docker-image' into 'master'

Update git and install git-lfs QA docker image

See merge request gitlab-org/gitlab-ce!22355

qa/Dockerfile

@@ -2,11 +2,21 @@ FROM ruby:2.4-stretch
 LABEL maintainer "Grzegorz Bizon <grzegorz@gitlab.com>"
 ENV DEBIAN_FRONTEND noninteractive
 
+##
+# Add support for stretch-backports
+#
+RUN echo "deb http://ftp.debian.org/debian stretch-backports main" >> /etc/apt/sources.list
+
 ##
 # Update APT sources and install some dependencies
 #
 RUN sed -i "s/httpredir.debian.org/ftp.us.debian.org/" /etc/apt/sources.list
-RUN apt-get update && apt-get install -y wget git unzip xvfb
+RUN apt-get update && apt-get install -y wget unzip xvfb
+
+##
+# Install some packages from backports
+#
+RUN apt-get -y -t stretch-backports install git git-lfs
 
 ##
 # Install Docker