[dock] Make raise_on_errors a member and use it in _exec

This allows to be coherent when running a command and docker is not
installed.

For example _discover_environment will fail even if raise_on_errors is
set to False if docker is not installed. This patch fixes that.
7 jobs for improve_raise_on_errors_handling in 42 minutes and 54 seconds (queued for 7 seconds)