Use env-var for post git step

templates/tools/dockerfile/test/cxx_squeeze_x64/Dockerfile.template

@@ -40,7 +40,8 @@
   RUN pip install argparse
 
   RUN wget ${openssl_fallback.base_uri + openssl_fallback.tarball}
-  ADD post-git-setup.sh /
+
+  ENV POST_GIT_STEP tools/dockerfile/test/cxx_squeeze_x64/post-git-setup.sh
 
   <%include file="../../run_tests_addons.include" args="skip_zookeeper=True"/>
   # Define the default command.

tools/jenkins/docker_run.sh

@@ -42,8 +42,6 @@ else
   cp -r "$EXTERNAL_GIT_ROOT/$RELATIVE_COPY_PATH"/* "/var/local/git/grpc/$RELATIVE_COPY_PATH"
 fi
 
-[ -e /post-git-setup.sh ] && /post-git-setup.sh
-
 if [ -x "$(command -v rvm)" ]
 then
   rvm use ruby-2.1

tools/jenkins/docker_run_tests.sh

@@ -43,10 +43,10 @@ chown $(whoami) $XDG_CACHE_HOME
 mkdir -p /var/local/git
 git clone --recursive /var/local/jenkins/grpc /var/local/git/grpc
 
-[ -e /post-git-setup.sh ] && /post-git-setup.sh
-
 mkdir -p reports
 
+$POST_GIT_STEP
+
 exit_code=0
 
 $RUN_TESTS_COMMAND || exit_code=$?