Drop media proxy same-domain default for base_url

Even more than with user uploads, a same-domain proxy setup bears
significant security risks due to serving untrusted content under
the main domain space.

A risky setup like that should never be the default.

docs/docs/configuration/howto_mediaproxy.md

@@ -6,7 +6,16 @@ With the `mediaproxy` function you can use nginx to cache this content, so users
 
 ## Activate it
 
-* Edit your nginx config and add the following location: 
+* Edit your nginx config and add the following location to your main server block:
+```
+location /proxy {
+        return 404;
+}
+```
+
+* Set up a subdomain for the proxy with its nginx config on the same machine
+  *(the latter is not strictly required, but for simplicity we’ll assume so)*
+* In this subdomain’s server block add
 ```
 location /proxy {
         proxy_cache akkoma_media_cache;
@@ -26,9 +35,9 @@ config :pleroma, :media_proxy,
       enabled: true,
       proxy_opts: [
             redirect_on_failure: true
-      ]
-      #base_url: "https://cache.akkoma.social"
+      ],
+      base_url: "https://cache.akkoma.social"
 ```
-If you want to use a subdomain to serve the files, uncomment `base_url`, change the url and add a comma after `true` in the previous line.
+You **really** should use a subdomain to serve proxied files; while we will fix bugs resulting from this, serving arbitrary remote content on your main domain namespace is a significant attack surface.
 
 * Restart nginx and Akkoma

lib/pleroma/web/media_proxy.ex

@@ -14,6 +14,8 @@ defmodule Pleroma.Web.MediaProxy do
 
   @cachex Pleroma.Config.get([:cachex, :provider], Cachex)
 
+  @mix_env Mix.env()
+
   def cache_table, do: @cache_table
 
   @spec in_banned_urls(String.t()) :: boolean()
@@ -144,8 +146,14 @@ defmodule Pleroma.Web.MediaProxy do
     if path = URI.parse(url_or_path).path, do: Path.basename(path)
   end
 
-  def base_url do
-    Config.get([:media_proxy, :base_url], Endpoint.url())
+  if @mix_env == :test do
+    def base_url do
+      Config.get([:media_proxy, :base_url], Endpoint.url())
+    end
+  else
+    def base_url do
+      Config.get!([:media_proxy, :base_url])
+    end
   end
 
   defp proxy_url(path, sig_base64, url_base64, filename) do