extract-docker: Remove leading slashes when removing an earlier copy

If extracting from an absolute path within the image, like
/opt/llvm-mingw, it will still be extracted into opt/llvm-mingw in
the local directory.

extract-docker.sh

@@ -12,5 +12,5 @@ fi
 image=$1
 dir=$2
 
-rm -rf $dir
+rm -rf $(echo $dir | sed 's,^/,,')
 docker run --rm $image tar -cf - $dir | tar -xvf -