Add a caveat for docker deployment in the config docs

docs/config.md

@@ -280,7 +280,7 @@ config :pleroma, :mrf_subchain,
 
 ## Pleroma.Web.Endpoint
 `Phoenix` endpoint configuration, all configuration options can be viewed [here](https://hexdocs.pm/phoenix/Phoenix.Endpoint.html#module-dynamic-configuration), only common options are listed here
-* `http` - a list containing http protocol configuration, all configuration options can be viewed [here](https://hexdocs.pm/plug_cowboy/Plug.Cowboy.html#module-options), only common options are listed here
+* `http` - a list containing http protocol configuration, all configuration options can be viewed [here](https://hexdocs.pm/plug_cowboy/Plug.Cowboy.html#module-options), only common options are listed here. For deployment using docker, you need to set this to `[ip: {0,0,0,0}, port: 4000]` to make pleroma accessible from other containers (such as your nginx server).
   - `ip` - a tuple consisting of 4 integers
   - `port`
 * `url` - a list containing the configuration for generating urls, accepts