support/docker/Dockerfile: unbreak 32bit support for x86-64

Fixes ttps://gitlab.com/buildroot.org/buildroot/-/jobs/9153298207

Commit 3bc94471ca8a ("support/docker: prepare for Aarch64 Docker images")
reworked the Dockerfile to only add the 32bit support when building for
x86-64, but it forgot to add a TARGETPLATFORM argument so the conditionals
never triggers.

Fix it by adding a TARGETPLATFORM argument as explained here:
https://forums.docker.com/t/build-multi-arch-images-with-different-commands-per-architecture-in-docker-file/134795

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

support/docker/Dockerfile

@@ -14,6 +14,9 @@ LABEL maintainer="Buildroot mailing list <buildroot@buildroot.org>" \
       vendor="Buildroot" \
 description="Container with everything needed to run Buildroot"
 
+# Architecture to build for
+ARG TARGETPLATFORM
+
 # We need tar >= 1.35
 ARG TAR_VERSION="1.35"