Add --frozen-lockfile in Docker example when using pnpm (#41291)

It seems that lock file is used when using npm and yarn, but pnpm doesn't, complete this code.

## Documentation / Examples

- [ ] Make sure the linting passes by running `pnpm lint`
- [ ] The "examples guidelines" are followed from [our contributing doc](https://github.com/vercel/next.js/blob/canary/contributing/examples/adding-examples.md)

examples/with-docker/Dockerfile

@@ -9,7 +9,7 @@ COPY package.json yarn.lock* package-lock.json* pnpm-lock.yaml* ./
 RUN \
   if [ -f yarn.lock ]; then yarn --frozen-lockfile; \
   elif [ -f package-lock.json ]; then npm ci; \
-  elif [ -f pnpm-lock.yaml ]; then yarn global add pnpm && pnpm i; \
+  elif [ -f pnpm-lock.yaml ]; then yarn global add pnpm && pnpm i --frozen-lockfile; \
   else echo "Lockfile not found." && exit 1; \
   fi