On my Linux boxes (Linux Mint), there is /etc/resolvconf/resolv.conf.d dir which has 3 files:  base, head, tail.

 

Put ‘nameserver 8.8.8.8’ into base (or tail) and it should always get added to resolv.conf every reboot.  (You might look at all 3 files, my favourite trick for this is:

 

more /etc/resolvconf/resolv.conf.d/*|less

 

and see what’s in them before you start.)