docker/ci: Add support for running cross platform build image (envoyp…

…roxy#28879)

Signed-off-by: Ryan Northey <[email protected]>

ci/run_envoy_docker.sh

@@ -63,6 +63,12 @@ else
           && sudo -EHs -u envoybuild bash -c 'cd /source && $*'")
 fi
 
+if [[ -n "$ENVOY_DOCKER_PLATFORM" ]]; then
+    echo "Setting Docker platform: ${ENVOY_DOCKER_PLATFORM}"
+    docker run --rm --privileged multiarch/qemu-user-static --reset -p yes
+    ENVOY_DOCKER_OPTIONS+=(--platform "$ENVOY_DOCKER_PLATFORM")
+fi
+
 # The IMAGE_ID defaults to the CI hash but can be set to an arbitrary image ID (found with 'docker
 # images').
 [[ -z "${IMAGE_ID}" ]] && IMAGE_ID="${ENVOY_BUILD_SHA}"