feat(nginx): Use builtin envsubst mechanics of nginx container

Part-of: <https://dev.funkwhale.audio/funkwhale/funkwhale/-/merge_requests/2547>

docker/nginx/conf.dev

@@ -81,6 +81,14 @@ server {
         proxy_pass   http://funkwhale-api/api/subsonic/rest/;
     }
 
+    location /media/__sized__/ {
+        alias /protected/media/__sized__/;
+    }
+
+    location /media/attachments/ {
+        alias /protected/media/attachments/;
+    }
+
     location /.well-known/ {
         include /etc/nginx/funkwhale_proxy.conf;
         proxy_pass   http://funkwhale-api;