feat: set up Mailpit SMTP server as part of our Docker Compose file (#4151)

* feat(labrinth): support STMP servers with no auth credentials

* feat: set up Mailpit SMTP server as part of our Docker Compose services swarm

* chore(docker-compose): fix healthcheck for mail service

* feat(docker-compose): enable SpamAssassin integration through Postmark

Unlike spinning up yet another container, this requires no
configuration, and is good and simple enough for a funny little feature
developers may occassionally use with non-confidential messages.

docker-compose.yml

@@ -58,6 +58,19 @@ services:
       interval: 3s
       timeout: 5s
       retries: 3
+  mail:
+    image: axllent/mailpit:v1.27
+    container_name: labrinth-mail
+    ports:
+      - '1025:1025'
+      - '8025:8025'
+    environment:
+      MP_ENABLE_SPAMASSASSIN: postmark
+    healthcheck:
+      test: ['CMD', 'wget', '-q', '-O/dev/null', 'http://localhost:8025/api/v1/info']
+      interval: 3s
+      timeout: 5s
+      retries: 3
   labrinth:
     profiles:
       - with-labrinth