Update docker stack configuration to match documentation changes

• Make the default storage db flavor "mirror"
• Gitignore the graph-replayer and content-replayer configs
• Migrate volume-dependent service placement constraints to labels

Related to T3829 Goes together with swh/devel/swh-docs!295 (closed)

Test Plan

It works on my machines™

---

Migrated from D6978 (view on Phabricator)