Allow to skip service registration through env variable

Introduces support for a new env variable SERVICE_IGNORE to skip service registration all together.
This is a quick way to avoid the inconsistencies described in issue #13 and it might be useful to explicitely avoid publishing services for a specific container.
Registration is ignored if SERVICE_IGNORE has any value. I thought it would be better than forcing "true" or something similar.
and this can check for `nil` and then do `continue` instead.