docs(mailcow): Document env var changes we're making

mailcow/README.md

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+# Overrides for mailcow-dockerized
+
+Provides files that are meant to be copied into your location of the [github.com/mailcow/mailcow-dockerized](https://github.com/mailcow/mailcow-dockerized) repo thereby customizing its behavior.
+
+yq 'del( .services.[].environment.[] | select(. == "TZ=*") ) | del(.. | select( (tag == "!!map" or tag =="!!seq") and length == 0)) | del( .services.[].restart )' /opt/containers/mailcow-dockerized/docker-compose.yml | yq --indent 4 ea '. as $item ireduce ({}; . * $item )' - /opt/containers/mailcow-dockerized/docker-compose.override.yml > /opt/containers/mailcow-dockerized/docker-compose.final.yml
+
+
+* Env variables
+    ```
+    yq 'del( .services.[].environment.[] | select(. == "TZ=*") ) | del(.. | select( (tag == "!!map" or tag =="!!seq") and length == 0))' ...
+    ```
+    From all `.services.[].environment.[]` lists remove items that match the expression `TZ=*`. Realistically this removes the `TZ` environment variable declaration and in some cases leaves the `environment` list empty. If the list now truly is empty delete it.
+
+    Given a file:
+    ```
+    services:
+        unbound-mailcow:
+            image: mailcow/unbound:1.16
+            environment:
+                - TZ=${TZ}
+            volumes:
+                - ./data/hooks/unbound:/hooks:Z
+    ```
+    Step one does:
+    ```
+    services:
+        unbound-mailcow:
+            image: mailcow/unbound:1.16
+       ---> environment:
+            volumes:
+                - ./data/hooks/unbound:/hooks:Z
+    ```
+    We detect that `environment` is now empty and delete it:
+    ```
+    services:
+        unbound-mailcow:
+            image: mailcow/unbound:1.16
+       ---> volumes:
+                - ./data/hooks/unbound:/hooks:Z
+    ```
+* Restart policy
+    ```
+    del( .services.[].restart ) ...
+    ```
+    From all services delete restart policy declaration. We're defaulting to `unless-stopped` via the `extends:` directive.