Configurable db fixtures path

[dennisvang]

• Moved db fixtures dir from src/main/resources into project root
• Modified Dockerfile to copy the fixtures into the image
• Added configuration option for the (relational) db fixtures path
• Use bootstrap.enabled property to disable the populator by default, so ops person can decide when to load fixtures

This allows us to mount fixture files into the container at /fdp/fixtures/<filename> to override the defaults or extend them. For example, we can override the default users by mounting only that file:

services:
  fdp:
    ...
    volumes:
      - ./my_custom_user-accounts.json:/fdp/fixtures/0100_user-accounts.json:ro