Bake pnpm into runner image, fix config loading with --config flag

Pre-install pnpm 9.15.4 via corepack in the image so it doesn't
download every run. Use --config CLI flag instead of CONFIG_FILE
env var to ensure container.options volume mount is applied.

infra/kubernetes/platform/gitea-runner/Dockerfile.runner-image

@@ -7,4 +7,5 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
     docker.io \
   && curl -sL https://github.com/kubernetes-sigs/kustomize/releases/download/kustomize%2Fv5.6.0/kustomize_v5.6.0_linux_amd64.tar.gz \
     | tar xz -C /usr/local/bin \
+  && corepack enable && corepack prepare pnpm@9.15.4 --activate \
   && apt-get clean && rm -rf /var/lib/apt/lists/*