Add harness-safe Docker composer install helper

Makefile

@@ -77,6 +77,11 @@ phan:
 test: docker-build-22
 	./script/docker-ci-local.sh
 
+# Harness/dev convenience: install vendor/ without host PHP/composer.
+.PHONY: vendor-docker
+vendor-docker: docker-build-22
+	./script/docker-composer-install.sh
+
 # Deprecated: PHP 7.4 on Ubuntu 16.04 (ircmaxell/php-compiler:16.04-dev image often unavailable).
 .PHONY: test-legacy-16
 test-legacy-16: rebuild-changed

script/docker-composer-install.sh

@@ -0,0 +1,39 @@
+#!/usr/bin/env bash
+# Install vendor/ using the dev Docker image, safe on harness hosts where bind-mounts are empty.
+#
+# This script writes vendor/ into the host workspace by streaming it back as a tarball, so that
+# subsequent docker-exec / bootstrap gates can use the normal bind-mount fast path.
+#
+# Usage:
+#   ./script/docker-composer-install.sh
+set -euo pipefail
+cd "$(dirname "$0")/.."
+
+IMAGE="${PHP_COMPILER_DEV_IMAGE:-php-compiler:22.04-dev}"
+
+# shellcheck source=ci-docker-preflight.sh
+source "$(dirname "$0")/ci-docker-preflight.sh"
+ci_docker_preflight
+ci_docker_acquire_single_ci_lock
+
+# shellcheck source=ci-docker-run.sh
+source "$(dirname "$0")/ci-docker-run.sh"
+
+if ! docker image inspect "$IMAGE" >/dev/null 2>&1; then
+  echo "Building dev image $IMAGE (make docker-build-22)..."
+  make docker-build-22
+fi
+
+rm -rf vendor
+mkdir -p vendor
+
+# Stream vendor/ back to host so it persists between runs.
+tar -cf - --exclude='.git' --exclude='.llvm' . | ci_docker_run -i -w /compiler "$IMAGE" bash -lc "
+  set -euo pipefail
+  tar -xf -
+  composer install --ignore-platform-reqs --no-ansi --no-interaction --no-progress
+  ./script/apply-patches.sh >/dev/null || true
+  tar -cf - vendor
+" | tar -xf -
+
+test -f vendor/autoload.php