[PATCH 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.

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

 tools/docker/Dockerfile | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/tools/docker/Dockerfile b/tools/docker/Dockerfile
index 967ac89fbde..c9d794082c8 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 +12,10 @@ LABEL org.opencontainers.image.description=" This image is for building U-Boot i
 # Make sure apt is happy
 ENV DEBIAN_FRONTEND=noninteractive
 
+ARG TARGETPLATFORM
+ARG BUILDPLATFORM
+RUN echo "Building $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