Adding possibility of specifying listening interface

[feriniguez]

When docker "host" network is used and the host has more than one interface, it is desirable to select the interface you want ntp to be listening to.

This small change allows to set a new variable.

[cturra]

thanks for your patience with my review here... while i can appreciate this is an important feature for you, it deviates a bit away from the original intent, which is to expose as little as possible from the host system. network host is not something in this project, but instead just --publish: 123:123/udp.

i understand you have a need for this and would encourage you to keep you fork up to date with this main project repo. if others in the community think i am completely of the mark here, i'd be open to hearing it, so please feel free to comment on this PR.