Add certificate watcher for external certs to reload nginx

In case of TLS_FLAVOR=[mail,cert], the user supplies their own certificates.
However, since nginx is not aware of changes to these files, it cannot
reload itself e.g. when the certs get renewed.

To solve this, let’s add a small daemon in the place of
`letsencrypt.py`, which uses a flexible file-watching framework and
reloads nginx in the case the certificates change ….

core/nginx/Dockerfile

@@ -7,7 +7,7 @@ RUN apk add --no-cache \
 RUN pip3 install jinja2
 # Image specific layers under this line
 RUN apk add --no-cache certbot nginx nginx-mod-mail openssl curl \
- && pip3 install idna requests
+ && pip3 install idna requests watchdog
 
 COPY conf /conf
 COPY *.py /