use pm2 to guard and monitor the web service in docker file (#2238)

web/Dockerfile

 RUN ln -s /usr/share/zoneinfo/${TZ} /etc/localtime \
     && echo ${TZ} > /etc/timezone
 
+# global runtime packages
+RUN yarn global add pm2 \
+    && yarn cache clean
+
 WORKDIR /app/web
 COPY --from=builder /app/web/public ./public
 COPY --from=builder /app/web/.next/standalone ./
 COPY --from=builder /app/web/.next/static ./.next/static
 
 
+COPY docker/pm2.json ./pm2.json
 COPY docker/entrypoint.sh ./entrypoint.sh
 
 ARG COMMIT_SHA

web/docker/entrypoint.sh

 export NEXT_PUBLIC_SENTRY_DSN=${SENTRY_DSN}
 export NEXT_PUBLIC_SITE_ABOUT=${SITE_ABOUT}
 
-node ./server.js
+pm2 start ./pm2.json --no-daemon

web/docker/pm2.json

 {
-    "apps": [
-      {
-        "name": "WebApp",
-        "exec_mode": "cluster",
-        "instances": 1,
-        "script": "./node_modules/next/dist/bin/next",
-        "cwd": "/app/web",
-        "args": "start"
-      }
-    ]
-  }
+  "apps": [
+    {
+      "name": "dify-web",
+      "script": "/app/web/server.js",
+      "cwd": "/app/web",
+      "exec_mode": "cluster",
+      "instances": 2
+    }
+  ]
+}