Add Gerrit email account and send-email configuration

We'll have to store Gerrit's email credentials in plain text. Protecting
from outside connections using these in case they'd ever leak is tricky.
We match the account `gerrit@...` in `user.access` and then the local IP
address of the gerrit container in `gerrit-client.access`, only allowing
authenticated users (`permit_sasl_authenticated`) from this IP.

Alternatively, we could use `permit` which would allow the whole gerrit
container to send emails from `gerrit@...` without login. Then the setup
would also allow more efficient, plain-text communication between Gerrit
and Postfix. The password would still be needed to login to IMAP (assu-
ming no further, invasive changes).
diff --git a/docker-compose.yml b/docker-compose.yml
index cbabb0b..a120e6c 100644
--- a/docker-compose.yml
+++ b/docker-compose.yml
@@ -41,13 +41,18 @@
     build:
       context: .
       dockerfile: gerrit/Dockerfile
-    env_file: gerrit/environment
+    hostname: review.${SA_PUBLIC_DOMAIN_NAME}
+    env_file:
+      - gerrit/environment
+      - environment
     environment:
+      - GERRIT_USER_EMAIL=gerrit@${SA_PUBLIC_DOMAIN_NAME}
       - HTTPD_LISTEN_URL=proxy-https://*:8080/
     networks:
       - gerritnet
     volumes:
       - ./logs/gerrit/:/var/gerrit/logs/:rw
+      - ./mail/passwd/gerrit/:/var/gerrit/passwd/:rw
       - ./gerrit/etc/:/var/gerrit/etc/:rw
       - ./gerrit/db/:/var/gerrit/db/:rw
       - ./gerrit/git/:/var/gerrit/git/:rw
@@ -69,10 +74,13 @@
       - "465:465"  # ESMTP (implicit TLS)
       - "587:587"  # ESMTP (explicit TLS => STARTTLS)
       - "993:993"  # IMAP4 (implicit TLS)
+    networks:
+      - gerritnet
     volumes:
       - ./logs/mail-supervisor/:/var/log/supervisor/:rw
       - ./logs/mail/:/var/log/mail/:rw
       - ./certs/:/etc/letsencrypt/:ro
+      - ./mail/passwd/:/tmp/passwd/:ro
       - ./mail/data/:/var/mail/:rw
       - ./mail/state/:/var/mail-state/:rw
       - ./mail/config/:/tmp/docker-mailserver/:rw