Add Dokploy self-hosting deployment guide

[mirhamzaumer07]

Adds a step-by-step guide for self-hosting OpenReply on Dokploy, as an alternative to the Vercel + Railway setup in docs/setup.md.

Covers a few Dokploy/Nixpacks-specific gotchas I hit while deploying:

• The repo doesn't pin a Node version, and Nixpacks' default resolves to a version too old for Prisma 7 / Next.js 16 — includes a nixpacks.toml fix
• Dokploy has no separate Build/Start Command field like Vercel/Railway — these need to be set as NIXPACKS_BUILD_CMD / NIXPACKS_START_CMD env vars instead
• prisma migrate deploy can't run during the Docker build step (no network access to the DB at that point) — needs to run in the start command instead

[vercel]

@mirhamzaumer07 is attempting to deploy a commit to the diwenne's projects Team on Vercel.

A member of the Team first needs to authorize it.