chore(postfix): 配置 Postfix 的默认 TLS 证书

- 在 `postfix` 容器镜像中安装 `ssl-cert`。
- 为 `smtpd` 指定 snakeoil 证书和私钥路径，启用默认 TLS 配置。

deploy/postfix/Dockerfile

@@ -5,7 +5,7 @@ ENV DEBIAN_FRONTEND=noninteractive
 RUN --mount=type=cache,target=/var/cache/apt,sharing=locked \
     --mount=type=cache,target=/var/lib/apt/lists,sharing=locked \
     rm -f /etc/apt/apt.conf.d/docker-clean && \
-    apt-get update && apt-get install -y --no-install-recommends postfix postfix-sqlite ca-certificates rsyslog
+    apt-get update && apt-get install -y --no-install-recommends postfix postfix-sqlite ca-certificates rsyslog ssl-cert
 COPY main.cf /etc/postfix/main.cf
 COPY master.cf /etc/postfix/master.cf
 COPY sqlite-*.cf /etc/postfix/

deploy/postfix/main.cf

@@ -20,6 +20,8 @@ smtpd_recipient_restrictions = permit_mynetworks, permit_sasl_authenticated, rej
 smtpd_sasl_type = dovecot
 smtpd_sasl_path = inet:dovecot:12345
 smtpd_sasl_auth_enable = yes
+smtpd_tls_cert_file = /etc/ssl/certs/ssl-cert-snakeoil.pem
+smtpd_tls_key_file = /etc/ssl/private/ssl-cert-snakeoil.key
 smtpd_tls_security_level = may
 smtp_tls_security_level = may