Dashes in environmental value names are not compatible with some shells

[nazywam]

Some of karton services use the dash (-) character within section names.
This works fine for .ini files but can cause problems when configured using environmental values. [1]

Some of the solutions we could fix that:

• base all services on bash instead of sh - it seems that bash handles them correctly
• add a short pre-processor when loading values from env so that user can e.g. use a __ instead of a -

[1] kubernetes/kubernetes#16863

[nazywam]

I think this issue can be closed after adding a small warning in the https://karton-core.readthedocs.io/en/latest/service_configuration.html docs section