chore(deploy): 增强 Docker 构建与发布流程

- 新增 GitHub Actions Docker 工作流，自动执行前端检查、后端测试并发布 GHCR 镜像。
- 为单容器与多容器部署补充镜像配置、使用说明和环境变量示例。
- 优化各 Dockerfile 的构建缓存与镜像体积，提升构建速度并减少无关文件进入镜像。

deploy/docker-compose.stack.yml

@@ -2,6 +2,7 @@ version: "3.9"
 
 services:
   api:
+    image: ${LANQIN_API_IMAGE:-lanqin-email-api:local}
     build:
       context: ..
       dockerfile: deploy/api.Dockerfile
@@ -15,6 +16,7 @@ services:
     restart: unless-stopped
 
   web:
+    image: ${LANQIN_WEB_IMAGE:-lanqin-email-web:local}
     build:
       context: ..
       dockerfile: deploy/web.Dockerfile
@@ -33,6 +35,7 @@ services:
     restart: unless-stopped
 
   postfix:
+    image: ${LANQIN_POSTFIX_IMAGE:-lanqin-email-postfix:local}
     build:
       context: ./postfix
     env_file: .env
@@ -48,6 +51,7 @@ services:
     restart: unless-stopped
 
   dovecot:
+    image: ${LANQIN_DOVECOT_IMAGE:-lanqin-email-dovecot:local}
     build:
       context: ./dovecot
     env_file: .env
@@ -59,6 +63,7 @@ services:
     restart: unless-stopped
 
   opendkim:
+    image: ${LANQIN_OPENDKIM_IMAGE:-lanqin-email-opendkim:local}
     build:
       context: ./opendkim
     env_file: .env