Fix `ONCE` environment variable ignorance

[dieselburner]

Hi there,

Here's the MR to fix the issue I have faced with ONCE environment variable being ignored, when executed via Docker.

One important note: this changes docker-compose-watcher (or, in fact, compose-updater) default value of once to false, so that it would be possible to remove -once=0 from Dockerfile, but still keeping the old behavior:

CMD ["docker-compose-watcher", "-once=0", "-printSettings"]

The reason of removing this argument from CMD above is rather simple: due to code structure command line arguments takes precedence over environment variables, which I think is the right way to go. However, keeping -once=0 will prevent ability to override behavior of passing environment variable ONCE to Docker.

Basically, this MR makes everyone happy.

Few other small things irrelevant to original issue comes as a bonus.

[dieselburner]

@virtualzone I would appreciate if you could review this and merge, or provide comments what needs to be addressed.