Docker Build Improvements

[zupzup]

• Improve build times
  • Cache dependency fetching & compilation (e.g. https://www.lpalmieri.com/posts/fast-rust-docker-builds/)
• Make it easy to switch between running a client with fresh state (no identity set etc.) and an existing state (identity, contacts, bills etc.)
  • Currently, using docker run .. creates a fresh state on every run and docker-compose up retains it's state between runs, as long as the image isn't rebuilt, or the container removes (using docker-compose down)
  • It would be nice to have a simple way (e.g. with an optional volume, or different compose configs) to decide whether to start with a fresh state, or use (and keep!) an existing state around for testing