Varnish (VCL synthetic) VCL

vcl 4.1;

import blob;

backend default none;  # or your real backend

sub vcl_recv {
    if (regsub(req.url, "\?.*$", "") == "/pixel.gif") {
        return (synth(200, "OK"));
    }
}

sub vcl_synth {
    if (resp.status == 200 && regsub(req.url, "\?.*$", "") == "/pixel.gif") {
        set resp.http.Content-Type = "image/gif";
        set resp.http.Cache-Control = "no-store";
        set resp.body = blob.decode(BASE64, encoded="R0lGODlhAQABAIAAAP///wAAACH5BAEAAAAALAAAAAABAAEAAAICRAEAOw==");
        return (deliver);
    }
}