docker: Support building for multiple architectures

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>

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 \