When the chasquid docker container is restarted, entrypoint.sh will add
the hostname again, even if it is present.
This causes chasquid to fail to start due to the duplicated option
(`non-repeated field "hostname" is repeated`).
Thanks to Jaywann@github for finding and reporting this problem, on
https://github.com/albertito/chasquid/issues/16.
This patch fixes the issue by only adding the option if it isn't already
present.
This patch adds a new docker directory, which contains a Dockerfile plus
some additional configuration for creating a container that runs
chasquid+dovecot+letsencrypt.
It also updates the gitlab CI pipeline to automatically build and
publish an image on each commit.
This is experimental and likely to break.
