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_s390x.sh

@@ -9,7 +9,8 @@ export LC_ALL=C.UTF-8
 export HOST=s390x-linux-gnu
 export PACKAGES="python3-zmq"
 export CONTAINER_NAME=ci_s390x
-export CI_IMAGE_NAME_TAG="docker.io/s390x/ubuntu:24.04"
+export CI_IMAGE_NAME_TAG="docker.io/ubuntu:24.04"
+export CI_IMAGE_PLATFORM="linux/s390x"
 export TEST_RUNNER_EXTRA="--exclude rpc_bind,feature_bind_extra"  # Excluded for now, see https://github.com/bitcoin/bitcoin/issues/17765#issuecomment-602068547
 export RUN_FUNCTIONAL_TESTS=true
 export GOAL="install"