RPi: update doc (alpine version + variables)

docs/rpi_build.rst

@@ -5,15 +5,21 @@ Building for a Raspberry Pi
 
 The build does not need to be done on the Pi.
 
-To do so, go to ``tests/`` and call ``./build_arm.sh``, it will build all necessary images for arm.
+To do so, go to ``tests/`` and call ``./build_arm.sh``, it will build all
+necessary images for arm.
 
-To push the locally built images to a remote server, run ``./deploy_to_pi.sh``. Docker 18.09+ is needed to use ``-H ssh://<user>@<host>``.
+To push the locally built images to a remote server, run ``./deploy_to_pi.sh``.
+Docker 18.09+ is needed to use ``-H ssh://<user>@<host>``.
 
 Adjustments
 -----------
 
 ``build_arm.sh`` uses some variables passed as ``build-arg`` to docker-compose:
 
-- ``DISTRO``: is the main distro used (ie: alpine:3.8)
-- ``PHP_DISTRO``: is used for the ``rainloop`` and ``roundcube`` images
-- ``EDGE_DISTRO``: is used for ``radicale`` as edge has dulwich and radicale as packages
+- ``ALPINE_VER``: version of ALPINE to use
+- ``DISTRO``: is the main distro used. Dockerfiles are set on Alpine 3.10, and
+  build script overrides for ``balenalib/rpi-alpine:3.10``
+- ``QEMU``: Used by webmails dockerfiles. It will add ``qemu-arm-static`` only
+  if ``QEMU`` is set to ``arm``
+- ``ARCH``: Architecture to use for ``admin``, and ``webmails`` as their images
+  are available for those architectures.