Added CHRONY_EXTRA option for additional configuration

[WattsUp]

This feature allows special chrony configurations to be added.

Such as having a PTP reference clock and using docker-ntp to serve to ntp clients

[Mimikoo]

Voting for extra configurations.

[cturra]

thanks for your patience with my review here. while i understand there may be other chronyd configuration options that can be passed to this container, i would like to keep those purposeful. it feels like a catch-all environment variable is a bit of a shortcut.

using your example, if you're looking to add the ability to pass a device to the container, let's get a specific option for that. this way we can also ensure that during startup, any relevant checks, like validating permissions on the device(s).