ci: Supply platform argument to docker commands.

Later versions of docker require a `--platform` argument when building
and running from a platform-specific image that differs from the host
platform.

ci/test/00_setup_env.sh

@@ -68,3 +68,6 @@ export CI_BASE_PACKAGES=${CI_BASE_PACKAGES:-build-essential pkgconf curl ca-cert
 export GOAL=${GOAL:-install}
 export DIR_QA_ASSETS=${DIR_QA_ASSETS:-${BASE_SCRATCH_DIR}/qa-assets}
 export CI_RETRY_EXE=${CI_RETRY_EXE:-"retry --"}
+
+# The --platform argument used with `docker build` and `docker run`.
+export CI_IMAGE_PLATFORM=${CI_IMAGE_PLATFORM:-"linux"} # Force linux, but use native arch by default