feat(docker): add Docker container for the agent (salvage #1841) (#3668)

Adds a complete Docker packaging for Hermes Agent:
- Dockerfile based on debian:13.4 with all deps
- Entrypoint that bootstraps .env, config.yaml, SOUL.md on first run
- CI workflow to build, test, and push to DockerHub
- Documentation for interactive, gateway, and upgrade workflows

Closes #850, #913.

Changes vs original PR:
- Removed pre-created legacy cache/platform dirs from entrypoint
  (image_cache, audio_cache, pairing, whatsapp/session) — these are
  now created on demand by the application using the consolidated
  layout from get_hermes_dir()
- Moved docs from docs/docker.md to website/docs/user-guide/docker.md
  and added to Docusaurus sidebar

Co-authored-by: benbarclay <benbarclay@users.noreply.github.com>

docker/SOUL.md

@@ -0,0 +1,15 @@
+# Hermes Agent Persona
+
+<!--
+This file defines the agent's personality and tone.
+The agent will embody whatever you write here.
+Edit this to customize how Hermes communicates with you.
+
+Examples:
+  - "You are a warm, playful assistant who uses kaomoji occasionally."
+  - "You are a concise technical expert. No fluff, just facts."
+  - "You speak like a friendly coworker who happens to know everything."
+
+This file is loaded fresh each message -- no restart needed.
+Delete the contents (or this file) to use the default personality.
+-->