Docker: use a entrypoint script (#1227)

In favor of `/etc/bash.bashrc`, which is only read for interactive
shells.

docker/test_dockerimage.sh

@@ -5,7 +5,6 @@ if [ $EUID -eq 0 ]; then
   sudo -u nobody `which emcc` --version
 fi
 
-which llvm-ar
 which emsdk
 node --version
 npm --version