Merge pull request #519 from nicolasnoble/docker-clang

Renaming the docker image to grpc_clang, and optimizing the build a bit.

tools/dockerfile/grpc_clang/Dockerfile

@@ -23,7 +23,10 @@ RUN mv libcxx llvm/projects
 RUN mv libcxxabi llvm/projects
 
 RUN mkdir llvm-build
-RUN cd llvm-build && cmake ../llvm
+RUN cd llvm-build && cmake \
+  -DCMAKE_BUILD_TYPE:STRING=Release \
+  -DLLVM_TARGETS_TO_BUILD:STRING=X86 \
+  ../llvm
 RUN make -C llvm-build && make -C llvm-build install && rm -rf llvm-build
 
 CMD ["bash"]