[v6 08/12] docker: Support building for multiple architectures

[Tom Rini <trini@konsulko.com>]

From: Simon Glass <sjg@chromium.org>

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

Update the documentation as well.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@konsulko.com>
---
Changes in v6:
- None
---
 doc/build/docker.rst    | 23 +++++++++++++++++++++--
 tools/docker/Dockerfile |  9 +++++++++
 2 files changed, 30 insertions(+), 2 deletions(-)

diff --git a/doc/build/docker.rst b/doc/build/docker.rst
index 45659b3b89dc..5896dd5ac4a6 100644
--- a/doc/build/docker.rst
+++ b/doc/build/docker.rst
@@ -1,11 +1,30 @@
 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).
+
+You will need a multi-platform container, otherwise this error is shown::
+
+    ERROR: Multi-platform build is not supported for the docker driver.
+    Switch to a different driver, or turn on the containerd image store, and try again.
+
+You can add one with::
+
+    sudo docker buildx create --name multiarch --driver docker-container --use
+
+Building is supported on both amd64 (i.e. 64-bit x86) and arm64 machines. While
+both amd64 and arm64 happen in parallel, the non-native part will take
+considerably longer as it must use QEMU to emulate the foreign code.
+
+To build the image yourself::
 
 .. code-block:: bash
 
-    sudo docker build -t your-namespace:your-tag .
+    sudo docker buildx build --platform linux/arm64/v8,linux/amd64 -t your-namespace:your-tag .
 
 Or to use an existing container
 
diff --git a/tools/docker/Dockerfile b/tools/docker/Dockerfile
index bc8d1ebbb1e8..43cd37070e48 100644
--- a/tools/docker/Dockerfile
+++ b/tools/docker/Dockerfile
@@ -6,9 +6,18 @@ FROM ubuntu:jammy-20240808
 LABEL org.opencontainers.image.authors="Tom Rini <trini@konsulko.com>"
 LABEL org.opencontainers.image.description=" This image is for building U-Boot inside a container"
 
+# 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
+
 # Make sure apt is happy
 ENV DEBIAN_FRONTEND=noninteractive
 
+RUN echo "Building on $BUILDPLATFORM, for target $TARGETPLATFORM"
+
 # Add LLVM repository
 RUN --mount=type=cache,target=/var/cache/apt,sharing=locked \
     --mount=type=cache,target=/var/lib/apt,sharing=locked \
-- 
2.43.0