protoc-artifacts: Use ENTRYPOINT to enable devtoolset-1.1

ENTRYPOINT is used even when other commands are specified on the "docker
run" command line. This allows running one-off commands in the docker
image (especially combined with volume binding with the host) with the
correct environment variables.
diff --git a/protoc-artifacts/Dockerfile b/protoc-artifacts/Dockerfile
index 2644693..c346586 100644
--- a/protoc-artifacts/Dockerfile
+++ b/protoc-artifacts/Dockerfile
@@ -39,5 +39,7 @@
                    devtoolset-1.1-libstdc++-devel.i686 && \
     yum clean all
 
+COPY scl-enable-devtoolset.sh /var/local/
+
 # Start in devtoolset environment that uses GCC 4.7
-CMD ["scl", "enable", "devtoolset-1.1", "bash"]
+ENTRYPOINT ["/var/local/scl-enable-devtoolset.sh"]