Add example of overlays in image creation

Improve documentation by providing an example of debos overlay action to
apply customizations to images.

Signed-off-by: Walter Lozano <walter.lozano@collabora.com>

content/architecture/platform-guide.md

@@ -324,11 +324,6 @@ build of Debian-based operating systems. Debos uses tools like `debootstrap`
 already present in the environment and relies on virtualisation to securely do
 privileged operations without requiring root access.
 
-Additionally at this stage customizations can be applied by using overlays.
-This process allows the default content of the packages to be combined with
-custom modifications to provide the desired solution. A common case is to apply overlays to
-change some default system settings found in `/etc` such as default hostname.
-
 Ospacks and how they should be processed to generate images are defined through
 YAML files.
 
@@ -422,6 +417,29 @@ actions:
     file: ospack-armhf.tar.gz
 ```
 
+Additionally at this stage customizations can be applied by using overlays.
+This process allows the default content of packages to be combined with
+custom modifications to provide the desired solution. A common case is to apply
+overlays to change some default system settings found in `/etc` such as default
+hostname or default package configuration.
+
+As an example of this mechanism, the following section is used to customize the
+behaviour of `dpkg`
+
+```
+  - action: overlay
+    source: overlays/dpkg-exclusions
+```
+
+Thanks to this action, the contents of the `overlays/dpkg-exclusions`
+directory will be applied to the image, which in this case consist of
+the file:
+
+    etc/dpkg/dpkg.cfg.d/apertis-exclusions
+
+This file will be added to the rootfs, which in this instance will change
+the default behaviour of dpkg to suit the needs of the image
+
 Collections of images are built every night and published on the
 [deployable image hosting website](https://images.apertis.org), such that
 developers can always download the latest image to deploy it to a target device