"build" subcommand flag: --dry-run

[matthiasbeyer]

A --dry-run flag could be added to the "build" subcommand to do everything except executing the build, so that one can investigate what would happen.

[christophprokop]

We discussed that it would be more useful to just write the /script file without starting the whole build.
That would save the time when a lot of [system] dependencies would be installed before the interesting part actually begins.
This way we could have a look at the generated /script file without the need to wait for it.
The container would not be needed as well.