file mounting is fraught with problems wrt change notifications.
first, we must be carefule that the inode does not change.
second, changes outside container do not result in fs events inside the container.
haraka cache settings files and relies on fs events. So, even
though the file gets updated inside the container, haraka doesn't
see it.
https://github.com/docker/docker/issues/15793
Docker uses an embedded DNS server (127.0.0.11) for user defined networks (UDN).
With the latest releases of docker, specifying 127.0.0.1 as --dns makes the
containers resolve 127.0.0.1 _inside_ the container's networking namespace
(not sure how it worked before this).
The next idea was to only specify --dns-search=. but this does not work.
This makes docker setup the containers to use 127.0.0.1 (or 127.0.0.11 for UDN).
In my mind, the UDN case should work but doesn't (not sure why).
So, the solution is to simply go with no --dns or --dns-search. Sadly,
setting dns-search just at container level does not work either :/ Strangely,
docker run --network=cloudron --dns-search=. appimage # does not work
docker run --network=cloudron appimage # works if you manually remove search from /etc/resolv.conf
So clearly, something inside docker triggers when one of the dns* options is set.
This means that #130 has to be fixed at app level (For Go, this means to use the cgo resolver).
