Logging level of Docker executor is always INFO

A containerizer is constructed with the agent's flags. The Docker containerizer hands these down to e.g., also create flags for the executor with its dockerFlags helper function.

That helper sets a few flag members, but e.g., does not set the logging level which will at its default value INFO.

We should either consistently pass through the logging level to the executor (we probably should also audit what other flags are not properly propagated), or even better investigate providing a mechanism to control executor flags on a per-task basis.