Support for easier installation of certain VMODs

[Toflar]

Hey folks,

Thanks for the nice container. I was wondering if it would be possible to provide an easier way to install certain VMODs, mainly those that are official (https://github.com/varnish/varnish-modules).

I think it would make sense to provide something like RUN install-vmod.sh xkey as a simplification. The container would not really become a lot bigger by just this file but it would allow to make installation of the most commonly used VMODs in custom containers way easier as well as centralize efforts in keeping the instructions needed in a central place rather than having different people maintaining their own commands 😄

For example I just tried to install xkey for a few local tests but I'm completely new to the whole ecosystem so I have no clue what dependencies are needed etc.

[teohhanhui]

Hmm... Since there's now an official varnish Docker image. I think we should deprecate this one? It's been a long journey, but it's coming to an end.