mediaproxy: proxy network-path references

Discovered-by: snow
Fixes: https://akkoma.dev/AkkomaGang/akkoma/issues/900
(cherry picked from commit bbf974adc82d09a2099fb1234729131929307911)

priv/scrubbers/media_proxy.ex

@@ -17,6 +17,10 @@ defmodule Pleroma.HTML.Transform.MediaProxy do
     {"src", media_url}
   end
 
+  def scrub_attribute(:img, {"src", "//" <> target}) do
+    scrub_attribute(:img, {"src", "https://" <> target})
+  end
+
   def scrub_attribute(_tag, attribute), do: attribute
 
   def scrub({:img, attributes, children}) do