Bump next from 9.5.3 to 9.5.4 in /test/docker/app (#759)

Bumps [next](https://github.com/vercel/next.js) from 9.5.3 to 9.5.4.
- [Release notes](https://github.com/vercel/next.js/releases)
- [Changelog](https://github.com/vercel/next.js/blob/canary/release.js)
- [Commits](vercel/next.js@v9.5.3...v9.5.4)

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