[Buildroot] [git commit branch/2023.02.x] doc/manual: add documentation about docker registry on gitlab.com

[Peter Korsgaard]

commit: https://git.buildroot.net/buildroot/commit/?id=7e86856d69666ae506bde293942ae335dd35fb4e
branch: https://git.buildroot.net/buildroot/commit/?id=refs/heads/2023.02.x

Add a paragraph and an example about using the Buildroot image registry
hosted on gtilab.com, for people who want to build their own image based
on the offical one.

Signed-off-by: Thierry GUIBERT <thierry.guibert at croix-rouge.fr>
Signed-off-by: Yann E. MORIN <yann.morin.1998 at free.fr>
(cherry picked from commit d9228faa4dbda084168e3c83273e651c05b6b08b)
Signed-off-by: Peter Korsgaard <peter at korsgaard.com>
---
 docs/manual/adding-board-support.txt | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/docs/manual/adding-board-support.txt b/docs/manual/adding-board-support.txt
index cd90a5871f..b3a8802452 100644
--- a/docs/manual/adding-board-support.txt
+++ b/docs/manual/adding-board-support.txt
@@ -54,3 +54,19 @@ utils/docker-run script and inside it issue these commands:
  $ make +<boardname>_defconfig+
  $ make
 --------------------
+
+By default, Buildroot developers use the official image hosted on the
+https://gitlab.com/buildroot.org/buildroot/container_registry/2395076[gitlab.com
+registry] and it should be convenient for most usage. If you still want
+to build your own docker image, you can base it off the official image
+as the +FROM+ directive of your own _Dockerfile_:
+
+----
+FROM registry.gitlab.com/buildroot.org/buildroot/base:YYYYMMDD.HHMM
+RUN ...
+COPY ...
+----
+
+The current version _YYYYMMDD.HHMM_ can be found in the +.gitlab-ci.yml+
+file at the top of the Buildroot source tree; all past versions are
+listed in the aforementioned registry as well.