docker: Use supervisord to launch chasquid and dovecot

Today, we launch dovecot in the background and chasquid in the
foreground using sudo.

This means that dovecot failures won't propagate, and signals to the
container (e.g. to stop it) also don't get propagated to dovecot
(because it's in the background) or chasquid (because they don't go
beyond the sudo process).

Thanks to [Guiorgy@github](https://github.com/Guiorgy) for identifying
the problem, proposing alternatives, help debugging, and discussing this
in https://github.com/albertito/chasquid/pull/70.

docker/Dockerfile

@@ -27,7 +27,8 @@ RUN DEBIAN_FRONTEND=noninteractive \
 		chasquid \
 		dovecot-lmtpd dovecot-imapd dovecot-pop3d \
 		dovecot-sieve dovecot-managesieved \
-		acl libcap2-bin sudo certbot && \
+		supervisor \
+		acl libcap2-bin certbot && \
 	apt-get autoremove --purge -y -q && \
 	apt-get autoclean -y -q && \
 	rm -rf /var/lib/apt/lists/*
@@ -42,6 +43,7 @@ RUN setcap CAP_NET_BIND_SERVICE=+eip /usr/bin/chasquid
 # Copy docker-specific configurations.
 COPY docker/dovecot.conf /etc/dovecot/dovecot.conf
 COPY docker/chasquid.conf /etc/chasquid/chasquid.conf
+COPY docker/supervisord.conf /etc/supervisor/supervisord.conf
 
 # Copy utility scripts.
 COPY docker/add-user.sh docker/entrypoint.sh /