Docker: Add DB_SYNC_CONFIG to docker compose file

[sgillespie]

Description

Add new DB_SYNC_CONFIG environment variable to docker compose and add to the docs

Checklist

• [x] Commit sequence broadly makes sense
• [x] Commits have useful messages
• [ ] New tests are added if needed and existing tests are updated
• [ ] Any changes are noted in the changelog
• [ ] Code is formatted with fourmolu on version 0.10.1.0 (which can be run with scripts/fourmolize.sh)
• [x] Self-reviewed the diff

Migrations

• [ ] The pr causes a breaking change of type a,b or c
• [ ] If there is a breaking change, the pr includes a database migration and/or a fix process for old values, so that upgrade is possible
• [ ] Resyncing and running the migrations provided will result in the same database semantically

If there is a breaking change, especially a big one, please add a justification here. Please elaborate more what the migration achieves, what it cannot achieve or why a migration is not possible.