Make rvm install 32bit artifacts explicitly on i686

tools/dockerfile/grpc_artifact_centos6_x86/Dockerfile

@@ -32,7 +32,7 @@ RUN gpg --keyserver hkp://pool.sks-keyservers.net --recv-keys 409B6B1796C275462A
 RUN \curl -sSL https://get.rvm.io | bash -s stable
 
 # Install Ruby 2.6
-RUN /bin/bash -l -c "rvm install ruby-2.6"
+RUN /bin/bash -l -c "rvm install ruby-2.6 --32"
 RUN /bin/bash -l -c "rvm use --default ruby-2.6"
 RUN /bin/bash -l -c "echo 'gem: --no-document' > ~/.gemrc"
 RUN /bin/bash -l -c "echo 'export PATH=/usr/local/rvm/bin:$PATH' >> ~/.bashrc"