Rack Ruby

GIF = "R0lGODlhAQABAIAAAP///wAAACH5BAEAAAAALAAAAAABAAEAAAICRAEAOw==".unpack1("m0").freeze

HEADERS = {
  "content-type" => "image/gif",
  "content-length" => GIF.bytesize.to_s,
  "cache-control" => "no-store",
}.freeze

run lambda { |env|
  if env["REQUEST_METHOD"] == "GET" && env["PATH_INFO"] == "/pixel.gif"
    [200, HEADERS.dup, [GIF]]
  else
    [404, { "content-type" => "text/plain" }, ["Not Found"]]
  end
}