Bootstrat seeding refactoring
refactor the seeding process + other changes in the bootstrap script:
- seeding now performed outside the django container (and can be disabled with --noseed)
- seeding now generates a usable docker image for the 'sleep' webapp and uploads it to the registry
- add the --nostart option to stop all containers at the end of the bootstrap process
- add the --force option to skip confirmation before removing old external volumes
- ensure that the dev-nginx container is restarted at the end of bootstrap (because of possible IP change)