docs: update customizing kernel build steps

How to customizing the Talos kernel documentation update.

Signed-off-by: Serge Logvinov <serge.logvinov@sinextra.dev>

website/content/docs/v0.11/Guides/customizing-the-kernel.md

@@ -3,18 +3,47 @@ title: "Customizing the Kernel"
 description: ""
 ---
 
+The installer image contains [`ONBUILD`](https://docs.docker.com/engine/reference/builder/#onbuild) instructions that handle the following:
+
+- the decompression, and unpacking of the `initramfs.xz`
+- the unsquashing of the rootfs
+- the copying of new rootfs files
+- the squashing of the new rootfs
+- and the packing, and compression of the new `initramfs.xz`
+
+When used as a base image, the installer will perform the above steps automatically with the requirement that a `customization` stage be defined in the `Dockerfile`.
+
+Build and push your own kernel:
+
+ ```shell
+ git clone https://github.com/talos-systems/pkgs.git
+ cd pkgs
+ make kernel-menuconfig USERNAME=_your_github_user_name_
+
+ docker login ghcr.io --username _your_github_user_name_
+ make kernel USERNAME=_your_github_user_name_ PUSH=true
+ ```
+
+Using a multi-stage `Dockerfile` we can define the `customization` stage and build `FROM` the installer image:
+
 ```docker
 FROM scratch AS customization
 COPY --from=<custom kernel image> /lib/modules /lib/modules
 
-FROM docker.io/andrewrynhard/installer:latest
+FROM ghcr.io/talos-systems/installer:latest
 COPY --from=<custom kernel image> /boot/vmlinuz /usr/install/vmlinuz
 ```
 
+When building the image, the `customization` stage will automatically be copied into the rootfs.
+The `customization` stage is not limited to a single `COPY` instruction.
+In fact, you can do whatever you would like in this stage, but keep in mind that everything in `/` will be copied into the rootfs.
+
+To build the image, run:
+
 ```bash
-docker build --build-arg RM="/lib/modules" -t talos-installer .
+DOCKER_BUILDKIT=0 docker build --build-arg RM="/lib/modules" -t installer:kernel .
 ```
 
-> Note: You can use the `--squash` flag to create smaller images.
+> Note: buildkit has a bug [#816](https://github.com/moby/buildkit/issues/816), to disable it use `DOCKER_BUILDKIT=0`
 
 Now that we have a custom installer we can build Talos for the specific platform we wish to deploy to.