Switch the Slack bot's release image to a hardened base

Context

The Slack bot example (sdk/typescript/examples/slack-bot) now ships a real, CI-published, signed container image (ghcr.io/stacklok/mecatl/slack-bot, added in #1054). Its docker/bot.Dockerfile still builds on plain node:24-slim — not a hardened base. Raised by Samuele during #1054's review; deferred deliberately rather than done in that PR. Full reasoning is already written as a comment directly above the FROM line in docker/bot.Dockerfile.

Why it's not a one-line fix

The build currently runs corepack enable / pnpm install / pnpm run build inside the final image. A hardened runtime tag (minimal, distroless-style) almost certainly has neither a shell nor a package manager, so this needs a real multi-stage rewrite:

• Builder stage: a -dev/full variant (has the toolchain) — installs deps, builds dist/, prunes node_modules to production-only.
• Runtime stage: the minimal hardened tag — COPY in just dist/ + the pruned node_modules, run node dist/index.js directly. No corepack/pnpm/shell needed at runtime at all.

Recommendation: Chainguard over Docker Hardened Images

• cgr.dev/chainguard/node: free, no registry login to pull. This repo already depends on this vendor for every other released image — mecated/mecak8s/mecatui build on cgr.dev/chainguard/static (see .ko.yaml). Adopting it here is zero new vendor relationship, zero new CI credentials.
  • One wrinkle: only latest/latest-dev are pullable for free; a specific pinned version tag needs a Chainguard sales conversation. Workaround, already precedented in .ko.yaml: pin the digest of latest at build time, not the moving tag.
• Docker Hardened Images (docker.com/products/hardened-images): also genuinely free (Apache 2.0, unpaywalled catalog) and has a Node.js image, but pulling from dhi.io requires docker login dhi.io even on the free tier — a new credential that would need to be provisioned and stored as a GitHub secret in this repo's CI. Their "change one line in your Dockerfile" pitch doesn't mention that.

Scope

• Rewrite docker/bot.Dockerfile as multi-stage per above.
• Verify the resulting image still runs correctly (Socket Mode connects, gRPC to a real/mock mecated works) — not just that it builds.
• Update the CI comment/notes in .github/workflows/release.yml's publish-slack-bot job if the build context or Dockerfile path changes.

Not urgent, not blocking anything — tracked so it doesn't quietly disappear.