Fixes to allow running the development docker environment

[lemval]

Following the guidelines the development environment could not be started. This was due to the following issues:

• The command for starting the docker-based environment mentioned the wrong startup folder (it now also daemonizes the containers)
• A file to remove did not exists (this is now guarded)
• The sample data repository is no longer there

[CLAassistant]

All committers have signed the CLA.

[lemval]

Partly duplicate of https://github.com/minvws/nl-covid19-notification-app-backend/pull/26 !