use a local cache of nginx if available

bin/compile

@@ -8,11 +8,22 @@ cache_dir=$2
 env_dir=$3
 bp_dir=$(dirname $(dirname $0))
 
-version="1.8.0"
-nginx_tarball_url="https://s3-external-1.amazonaws.com/heroku-buildpack-ruby/nginx/cedar-14/nginx-$version.tgz"
+fetch_nginx_tarball() {
+    local version="1.8.0"
+    local tarball_file="nginx-$version.tgz"
+    local stack="cedar-14"
+    local nginx_tarball_url="https://s3-external-1.amazonaws.com/heroku-buildpack-ruby/nginx/$stack/nginx-$version.tgz"
+    local dest_path="$cache_dir/$stack/$tarball_file"
+
+    if [ -f "$dest_path" ]; then
+        echo -n "cat $dest_path"
+    else
+        echo -n "curl -L $nginx_tarball_url"
+    fi
+}
 
 mkdir -p $build_dir/bin
-curl -L $nginx_tarball_url | tar xzv -C $build_dir/bin
+$(fetch_nginx_tarball) | tar xzC $build_dir/bin
 nginx_version=$($build_dir/bin/nginx-$STACK -V 2>&1 | head -1 | awk '{ print $NF }')
 cp -a $bp_dir/scripts/boot $build_dir/bin/boot
 cp -a $bp_dir/scripts/config $build_dir/bin/config