Enable gzip compression in Varnish

varnish.vcl

sub vcl_recv {
    ...
    if (req.http.Accept-Encoding) {
      if (req.url ~ "\.(jpeg|jpg|png|gif|gz|tgz|bz2|tbz|mp3|ogg|swf|flv)$") {
        unset req.http.Accept-Encoding;
      } elsif (req.http.Accept-Encoding ~ "gzip") {
        set req.http.Accept-Encoding = "gzip";
      } elsif (req.http.Accept-Encoding ~ "deflate" &&
        req.http.user-agent !~ "MSIE") {
        set req.http.Accept-Encoding = "deflate";
      } else {
        unset req.http.Accept-Encoding;
      }
    }
    ...
}

sub vcl_backend_response {
  ...
  if (beresp.http.url ~ "\.(jpg|png|gif|gz|tgz|bz2|tbz|mp3|ogg|swf)$") {
    set beresp.do_gzip = false;
  }
  else {
    set beresp.do_gzip = true;
    set beresp.http.X-Cache = "ZIP";
  }
  ...
}