[PATCH v2 3/9] docker: Support building for multiple architectures

[Simon Glass]

Add instructions on how to build the file for multiple architectures.
Add a message indicating what is happening.

Update the documentation as well. Drop the 'sudo' since these should not
be needed if Docker is correctly configured.

Signed-off-by: Simon Glass <sjg at chromium.org>
---

Changes in v2:
- Update docs also
- Add comments to the ARG variables

 doc/build/docker.rst    | 16 +++++++++++++---
 tools/docker/Dockerfile | 12 ++++++++++++
 2 files changed, 25 insertions(+), 3 deletions(-)

diff --git a/doc/build/docker.rst b/doc/build/docker.rst
index 45659b3b89d..b854fad43a9 100644
--- a/doc/build/docker.rst
+++ b/doc/build/docker.rst
@@ -1,14 +1,24 @@
 GitLab CI / U-Boot runner container
 ===================================
 
-In order to have a reproducible and portable build environment for CI we use a container for building in.  This means that developers can also reproduce the CI environment, to a large degree at least, locally.  This file is located in the tools/docker directory.  To build the image yourself
+In order to have a reproducible and portable build environment for CI we use a container for building in.  This means that developers can also reproduce the CI environment, to a large degree at least, locally.  This file is located in the tools/docker directory.
+
+The docker image supports both amd64 and arm64. Ensure that the
+'docker-buildx' Debian package is installed (or the equivalent on another
+distribution).
+
+Building is currently only supports on an amd64 machine (i.e. 64-bit x86). While
+both amd64 and arm64 happen in parallel, the arm64 part will take considerably
+longer as it must use QEMU to emulate the arm64 code.
+
+To build the image yourself::
 
 .. code-block:: bash
 
-    sudo docker build -t your-namespace:your-tag .
+    docker buildx build --platform linux/arm64/v8,linux/amd64 -t your-namespace:your-tag .
 
 Or to use an existing container
 
 .. code-block:: bash
 
-    sudo docker pull trini/u-boot-gitlab-ci-runner:jammy-20240227-14Mar2024
+    docker pull trini/u-boot-gitlab-ci-runner:jammy-20240227-14Mar2024
diff --git a/tools/docker/Dockerfile b/tools/docker/Dockerfile
index 967ac89fbde..540718df062 100644
--- a/tools/docker/Dockerfile
+++ b/tools/docker/Dockerfile
@@ -2,6 +2,9 @@
 # This Dockerfile is used to build an image containing basic stuff to be used
 # to build U-Boot and run our test suites.
 
+# Build with (for example):
+# docker buildx build --platform linux/arm64/v8,linux/amd64 --tag sjg20/u-boot-gitlab-ci-runner-multiarch:jammy-20240808-03Nov2024 .
+
 FROM ubuntu:jammy-20240808
 LABEL org.opencontainers.image.authors="Tom Rini <trini at konsulko.com>"
 LABEL org.opencontainers.image.description=" This image is for building U-Boot inside a container"
@@ -9,6 +12,15 @@ LABEL org.opencontainers.image.description=" This image is for building U-Boot i
 # Make sure apt is happy
 ENV DEBIAN_FRONTEND=noninteractive
 
+# Used by docker to set the target platform: valid values are linux/arm64/v8
+# and linux/amd64
+ARG TARGETPLATFORM
+
+# Used by docker to set the build platform: the only valid value is linux/amd64
+ARG BUILDPLATFORM
+
+RUN echo "Building on $BUILDPLATFORM, for target $TARGETPLATFORM"
+
 # Add LLVM repository
 RUN apt-get update && apt-get install -y gnupg2 wget xz-utils && rm -rf /var/lib/apt/lists/*
 RUN wget -O - https://apt.llvm.org/llvm-snapshot.gpg.key | apt-key add -
-- 
2.34.1