The script is now well-structured (with functions!) and can be used
to reset a single container, without requiring a bootstrap of the
whole environment.

    usage: $0 [CONTAINER ...]

    The bootstrap script initialises the environment and the selected
    containers (or by default all the containers).

    If a container is already initialised, then the script asks for
    confirmation before purging its data (to bootstrap it again).

This commit also merges the 'prepare.sh' and 'init-container' scripts
into 'bootstrap'.