Starting with Docker 20.10, `--hostname` seems to have the side-effect
of making Docker's internal DNS server resolve said hostname to the IP
address of the container.

Because we were giving the mailer service a hostname of `matrix.DOMAIN`,
all requests destined for `matrix.DOMAIN` originating from other
services on the container network were resolving to `matrix-mailer`.
This is obviously wrong.

Initially reported here: https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/748

We normally try to not use the public hostname (and IP address) on the
container network and try to make services talk to one another locally,
but it sometimes could happen.

With this, we use a `matrix-mailer` hostname for the matrix-mailer
container. My testing shows that it doesn't cause any trouble with
email deliverability.