explicitly set reverse proxy upstream to IPv4

since Pleroma.Web.Endpoint binds on IPv4 only and `localhost.` resolves to
  [::0] on some systems

  fixes #930.

installation/caddyfile-pleroma.example

@@ -10,7 +10,9 @@ example.tld  {
 
   gzip
 
-  proxy / localhost:4000 {
+  # this is explicitly IPv4 since Pleroma.Web.Endpoint binds on IPv4 only
+  # and `localhost.` resolves to [::0] on some systems: see issue #930
+  proxy / 127.0.0.1:4000 {
     websocket
     transparent
   }

installation/pleroma-apache.conf

@@ -58,8 +58,10 @@ CustomLog ${APACHE_LOG_DIR}/access.log combined
     RewriteRule /(.*) ws://localhost:4000/$1 [P,L]
 
     ProxyRequests off
-    ProxyPass / http://localhost:4000/
-    ProxyPassReverse / http://localhost:4000/
+    # this is explicitly IPv4 since Pleroma.Web.Endpoint binds on IPv4 only
+    # and `localhost.` resolves to [::0] on some systems: see issue #930
+    ProxyPass / http://127.0.0.1:4000/
+    ProxyPassReverse / http://127.0.0.1:4000/
 
     RequestHeader set Host ${servername}
     ProxyPreserveHost On

installation/pleroma.nginx

@@ -69,7 +69,9 @@ server {
         proxy_set_header Connection "upgrade";
         proxy_set_header Host $http_host;
 
-        proxy_pass http://localhost:4000;
+	# this is explicitly IPv4 since Pleroma.Web.Endpoint binds on IPv4 only
+	# and `localhost.` resolves to [::0] on some systems: see issue #930
+        proxy_pass http://127.0.0.1:4000;
 
         client_max_body_size 16m;
     }