Better incorporate the new, great documentation (#1878)

docs/content/config/advanced/override-defaults/user-patches.md

@@ -0,0 +1,43 @@
+---
+title: 'Custom User Changes & Patches | Scripting'
+---
+
+If you'd like to change, patch or alter files or behavior of `docker-mailserver`, you can use a script.
+
+In case you cloned this repository, you can copy the file `user-patches.sh.dist` under `config/` with `#!sh cp config/user-patches.sh.dist config/user-patches.sh` in order to create the `user-patches.sh` script. In case you are managing your directory structure yourself, create a `config/` directory and the `user-patches.sh` file yourself.
+
+``` sh
+# 1. Either create the config/ directory yourself
+#    or let docker-mailserver create it on initial
+#    startup
+~/somewhere $ mkdir config && cd config
+
+# 2. Create the user-patches.sh and edit it
+~/somewhere/config $ touch user-patches.sh
+~/somewhere/config $ vi user-patches.sh
+```
+
+The contents could look like this
+
+``` sh
+#! /bin/bash
+
+sed -i -E 's|(log_level).*|\1 = -1;|g' /etc/amavis/conf.d/49-docker-mailserver
+
+cat >/etc/amavis/conf.d/50-user << "END"
+use strict;
+
+$undecipherable_subject_tag = undef;
+$admin_maps_by_ccat{+CC_UNCHECKED} =  undef;
+
+#------------ Do not modify anything below this line -------------
+1;  # ensure a defined return
+END
+
+...
+```
+
+And you're done. The user patches script runs right before starting daemons. That means, all the other configuration is in place, so the script can make final adjustments.
+
+!!! note
+    Many "patches" can already be done with the Docker Compose-/Stack-file. Adding hostnames to `/etc/hosts` is done with the `#!yaml extra_hosts:` section, `sysctl` commands can be managed with the `#!yaml sysctls:` section, etc.