Consolidate on using docker compose in dev mode

[alecthomas]

We now have two paths to creating Docker containers - our own imperative functions, and one using docker compose. We should consolidate on one, and docker compose seems like the way to go.

We can also eliminate our customer "readiness" function and replace it with health checks, eg.

    healthcheck:
      test: ["CMD-SHELL", "pg_isready"]
      interval: 1s
      timeout: 60s
      retries: 60
      start_period: 80s

[matt2e]

• Centralise logic so others call into it
• Use flocks to prevent multiple of the same type at once (rather than locks)