doc: add build subtree

docs/build/custom_images.md

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+---
+layout: default
+title: Custom images
+parent: Build system
+---
+
+# Custom images
+
+You can create custom images using [Image Builder](https://openwrt.org/docs/guide-user/additional-software/imagebuilder).
+
+Download the image builder from:
+```
+https://updates.nethsecurity.nethserver.org/<version>/targets/x86/64/nethsecurity-imagebuilder-<version>-x86-64.Linux-x86_64.tar.xz
+```
+Replace `version` with the latest release, like `23.05.06`.
+
+## Using Image Builder inside CentOS 7
+
+Install the dependencies:
+```
+yum install perl-Thread-Queue devtoolset-7-make git unzip bzip2
+```
+
+Upload the builder inside the system and prepare it:
+```
+mkdir builder; tar xvf nethsecurity-imagebuilder-22.03.0-x86-64.Linux-x86_64.tar.xz -C builder
+cd builder
+sed -i '/logd \\/d' include/target.mk
+```
+Note: OpenWrt build system assumes logd is always installed, but NethSecurity image does not ship it.
+The `sed` command ensure logd is not bundled inside the image.
+
+Prepare the files directory:
+```
+wget https://github.com/NethServer/nethsecurity/archive/refs/heads/master.tar.gz
+tar xvf master.tar.gz nethsecurity-master/files/
+```
+
+To include custom files, add them inside `nethsecurity-master/files` directory
+with the full path, like:
+```
+echo "hello" > file/root/goofy
+```
+
+Start the build with all included packages:
+```
+source /opt/rh/devtoolset-7/enable
+make image  FILES="nethsecurity-master/files/" PACKAGES="$(grep "Package: " packages/Packages | cut -d ':' -f 2 | tr '\n' ' ')"
+```