feat: split named volumes and bind mounts into separate methods

Added an internal Dockertest ID which is used for all resources that the test instance creates (volumes, networks etc). The Composition is now responsible for creating volumes, the DockerTest object now only keeps track of volume IDs for cleanup.

This MR also resolves our volume problem in a parallel test setting, previously all parallel tests would share the same volumes. Now all volumes are suffixed with their respective Dockertest ID.

Depends on: !28 (closed)