Set leaf dockerfiles to clean C library before rebuilding

tools/dockerfile/grpc_node/Dockerfile

@@ -7,6 +7,7 @@ RUN cd /var/local/git/grpc \
   && git submodule update --init --recursive
 
 # Install the C core.
+RUN make clean -C /var/local/git/grpc
 RUN make install_c -C /var/local/git/grpc
 
 RUN cd /var/local/git/grpc/src/node && npm install && node-gyp rebuild

tools/dockerfile/grpc_php/Dockerfile

@@ -5,6 +5,7 @@ RUN cd /var/local/git/grpc \
   && git pull --recurse-submodules \
   && git submodule update --init --recursive
 
+RUN make clean -C /var/local/git/grpc
 RUN make install_c -j12 -C /var/local/git/grpc
 
 RUN cd /var/local/git/grpc/src/php/ext/grpc && git pull && phpize

tools/dockerfile/grpc_ruby/Dockerfile

@@ -10,6 +10,7 @@ RUN cd /var/local/git/grpc \
 RUN touch /var/local/git/grpc/include/grpc/support/string.h
 
 # Build the C core.
+RUN make clean -C /var/local/git/grpc
 RUN make install_c -C /var/local/git/grpc
 
 # Build ruby gRPC and run its tests