fix(slack-bot): harden release image

[JAORMX]

Summary

• build the published Slack bot image through hardened, digest-pinned Chainguard builder and runtime stages
• ship only compiled output and production dependencies under a non-root, shellless runtime
• add CI packaging, runtime-entrypoint, and hardening checks

Verification

• task lint
• task test
• task docs
• task site:build
• task lint:actions
• Docker image build and runtime checks

Fixes #1085

[jhrozek]

Panel review (Spec / Standards / Test-adequacy / Domain specialists) on PR #1182. Solid hardening work overall (multi-stage build, digest-pinned Chainguard base matching .ko.yaml convention, non-root, shell removed, no baked-in secrets) — one blocker on unmet issue scope, plus a few important/advisory findings below.

[jhrozek]

Follow-up panel review after the latest fix commit (919d3117e).

The blocker from the previous review is fixed correctly: the new CI step spawns a real offline mecated --mock daemon and runs the actual built/hardened image's MecatlBridge against it over gRPC, asserting a genuine response — closing the "never tested against the built image" gap, with an honest comment scoping out Socket Mode itself (which needs live Slack credentials, out of reach for CI). Verified no new issues from this addition:

• The step lives in the slack-bot-example job on ubuntu-24.04 — a real Linux runner, so docker run --network host behaves as intended, not Docker Desktop's VM-isolated networking.
• The trap ... EXIT is registered immediately after backgrounding the daemon, minimizing any leak window; the readiness poll loop is correctly bounded (10s) and fails fast via kill -0 if the daemon dies early.
• The hardcoded mock-provider response string is a pre-existing coupling already shared by two other test files, not a new fragility introduced here.
• No new security surface — --network host only affects this ephemeral CI job, and the mock daemon needs no credentials.

All three remaining advisories are also fixed correctly: Dependabot gained a docker ecosystem entry scoped to the right directory, .dockerignore now excludes **/.npmrc, and the builder FROM line dropped the moving latest-dev tag, keeping only the digest (matching .ko.yaml's convention).

Approving.

[jhrozek]

Re-reviewed the changes since my last approval — the streaming/cancel/stuck-auth-eviction work addressing the other review round looks correct and is well covered by new tests. The one failing check (Test (race root-a), internal/app) is a pre-existing flake on main unrelated to this change, not something introduced here. LGTM.