Merge pull request #5430 from jtattermusch/ruby_distribtest_centos6_fix

Fix ruby distribtest on centos6

tools/dockerfile/distribtest/ruby_centos6_x64/Dockerfile

@@ -35,7 +35,9 @@ RUN yum install -y tar which
 
 # Install rvm
 RUN gpg --keyserver hkp://keys.gnupg.net --recv-keys 409B6B1796C275462A1703113804BB82D39DC0E3
-RUN \curl -sSL https://get.rvm.io | bash -s stable --ruby
+# Running the installation twice to work around docker issue when using overlay.
+# https://github.com/docker/docker/issues/10180
+RUN (curl -sSL https://get.rvm.io | bash -s stable --ruby) || (curl -sSL https://get.rvm.io | bash -s stable --ruby)
 
 RUN /bin/bash -l -c "echo '. /etc/profile.d/rvm.sh' >> ~/.bashrc"
 RUN /bin/bash -l -c "gem install --update bundler"