prepare.sh: use errtrace to trigger trap from function

[nicki-krizek]

When using -e/-o errexit without the -E/-o errtrace option, trap won't
be triggered if a command fails inside a function. In this case, all
relevant commands are executed inside start_container() and
install_dependencies() functions.

When the error is properly propagated, the trap error handling actually
works and the gitlab runner will attempt to re-try to start-up the
container since it correctly detects that the job failed during the
preparation stage.

See https://stackoverflow.com/a/35800451 for more details about -e -E
interaction

[jonasbb]

Thanks, that looks good.

Btw. I really like the openness of CZ NIC like open source contributions or presentations at OARC :)