use apptainer rather than Docker for local getting started docs

[Roland Haas]

Apptainer (https://apptainer.org/docs/admin/main/installation.html#install-unprivileged-from-pre-built-binaries) can be installed and run without root privileges and also integrates more nicely into the system by eg. making $HOME available and running as the user’s uid in the container.

It would seem to be more suitable than docker (runs as root, files created are owned by root) for a tutorial.