Added persistent caching for npm dependencies in Jenkins Dockerfiles

tools/jenkins/build_docker_and_run_tests.sh

@@ -66,6 +66,7 @@ docker run \
   -i $TTY_FLAG \
   -v "$git_root:/var/local/jenkins/grpc" \
   -v /tmp/ccache:/tmp/ccache \
+  -v /tmp/npm-cache:/tmp/npm-cache \
   -v /tmp/xdg-cache-home:/tmp/xdg-cache-home \
   -v /var/run/docker.sock:/var/run/docker.sock \
   -v $(which docker):/bin/docker \

tools/jenkins/grpc_jenkins_slave/Dockerfile

@@ -101,7 +101,7 @@ ENV NUGET mono /var/local/NuGet.exe
 # Install nvm
 RUN touch .profile
 RUN curl -o- https://raw.githubusercontent.com/creationix/nvm/v0.25.4/install.sh | bash
-RUN /bin/bash -l -c "nvm install 0.12"
+RUN /bin/bash -l -c "nvm install 0.12 && npm config set cache /tmp/npm-cache"
 
 ##################
 # Ruby dependencies

tools/jenkins/grpc_jenkins_slave_32bits/Dockerfile

@@ -101,7 +101,7 @@ ENV NUGET mono /var/local/NuGet.exe
 # Install nvm
 RUN touch .profile
 RUN curl -o- https://raw.githubusercontent.com/creationix/nvm/v0.25.4/install.sh | bash
-RUN /bin/bash -l -c "nvm install 0.12"
+RUN /bin/bash -l -c "nvm install 0.12 && npm config set cache /tmp/npm-cache"
 
 ##################
 # Ruby dependencies