support/docker: prepare for Aarch64 Docker images

With the new 'buildx' command using 'BuildKit' since Docker 23.0 [1],
it become easy to build the same image for several architectures [2].
The only requirement is to use a Dockerfile without any architecture
specific definition.

Since our current Dockefile contains already some i386 specific
packages (g++-multilib,libc6:i386) to provide x86 32bits support,
we have to install them conditionally.

Update the build process described in the Dockerfile accordingly.

For now, Aarch64 hosts can't be used by the Buildroot testsuite (yet)
since the Bootlin external toolchain (used by default) currently only
support x86_64 hosts.

[1] https://docs.docker.com/engine/release-notes/23.0/
[2] https://www.docker.com/blog/multi-arch-build-and-images-the-simple-way/
[3] https://www.docker.com/blog/multi-arch-build-what-about-gitlab-ci/

Signed-off-by: Romain Naour <romain.naour@smile.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

support/docker/Dockerfile

@@ -1,7 +1,9 @@
 # This Dockerfile generates the docker image that gets used by Gitlab CI
-# To build it (YYYYMMDD.HHMM is the current date and time in UTC):
-#   docker build -t registry.gitlab.com/buildroot.org/buildroot/base:YYYYMMDD.HHMM support/docker
-#   docker push registry.gitlab.com/buildroot.org/buildroot/base:YYYYMMDD.HHMM
+# To build it for arm64 and amd64 (YYYYMMDD.HHMM is the current date and time in UTC):
+#   docker buildx create --use
+#   docker buildx build --platform linux/amd64,linux/arm64/v8 \
+#       -t registry.gitlab.com/buildroot.org/buildroot/base:YYYYMMDD.HHMM \
+#       --push support/docker
 
 # We use a specific tag for the base image *and* the corresponding date
 # for the repository., so do not forget to update the apt-sources.list
@@ -21,9 +23,20 @@ ENV DEBIAN_FRONTEND noninteractive
 # This repository can be a bit slow at times. Don't panic...
 COPY apt-sources.list /etc/apt/sources.list
 
+# Install 32bit variant on x86_64 image.
+RUN if [ "$TARGETPLATFORM" = "linux/amd64" ]; then \
+    dpkg --add-architecture i386; \
+fi
+
 # The container has no package lists, so need to update first
-RUN dpkg --add-architecture i386 && \
-    apt-get -o APT::Retries=3 update -y
+RUN apt-get -o APT::Retries=3 update -y
+
+RUN if [ "$TARGETPLATFORM" = "linux/amd64" ]; then \
+    apt-get -o APT::Retries=3 install -y --no-install-recommends \
+        g++-multilib \
+        libc6:i386; \
+fi
+
 RUN apt-get -o APT::Retries=3 install -y --no-install-recommends \
         bc \
         build-essential \
@@ -35,9 +48,8 @@ RUN apt-get -o APT::Retries=3 install -y --no-install-recommends \
         cvs \
         file \
         flake8 \
-        g++-multilib \
+        g++ \
         git \
-        libc6:i386 \
         libncurses5-dev \
         locales \
         mercurial \