NestJSのappをGCPにデプロイしたときに出たエラーの話

##はじめに
NestCLIで hello world! を表示するnestjsのappの雛形を作成した。ローカルで開くとしっかり表示されるが、GCPにデプロイしてサイトにアクセスすると、500 server errorが出てしまった。

解決策

package.json

{
  "name": "attendance-server",
  "version": "0.0.1",
  "description": "",
  "author": "",
  "private": true,
  "license": "UNLICENSED",
  "engines": {
    "node": ">=8.0.0"
  },
  "main": "dist/main.js",
  "scripts": {
    "prepare": "npm run build",
    "pretest": "npm run build",
    "build": "tsc -p tsconfig.build.json",
    "deploy": "gcloud app deploy",
    "lint": "tslint -p tsconfig.json -c tslint.json",
    "start": "node ./dist/main.js",
    "start:dev": "nodemon",
    "start:debug": "nodemon --config nodemon-debug.json",
    "test": "repo-tools test app -- dist/main.js",
    "gcp-build": "npm run build"
  },
  "dependencies": {
    "@nestjs/common": "^7.0.9",
    "@nestjs/core": "^7.0.9",
    "@nestjs/mongoose": "^6.4.0",
    "@nestjs/passport": "^7.0.0",
    "@nestjs/platform-express": "^7.0.0",
    "@nestjs/typeorm": "^7.0.0",
    "express": "^4.17.1",
    "mongoose": "^5.9.11",
    "mysql": "^2.18.1",
    "reflect-metadata": "^0.1.13",
    "rimraf": "^3.0.2",
    "rxjs": "^6.5.4",
    "typeorm": "^0.2.24"
  },
  "devDependencies": {
    "@nestjs/cli": "^7.0.0",
    "@nestjs/schematics": "^7.0.0",
    "@nestjs/testing": "^7.0.0",
    "@types/express": "^4.17.3",
    "@types/jest": "25.1.4",
    "@types/node": "^13.9.1",
    "@types/supertest": "^2.0.8",
    "@typescript-eslint/eslint-plugin": "^2.23.0",
    "@typescript-eslint/parser": "^2.23.0",
    "eslint": "^6.8.0",
    "eslint-config-prettier": "^6.10.0",
    "eslint-plugin-import": "^2.20.1",
    "jest": "^25.1.0",
    "prettier": "^1.19.1",
    "supertest": "^4.0.2",
    "ts-jest": "25.2.1",
    "ts-loader": "^6.2.1",
    "ts-node": "^8.6.2",
    "tsconfig-paths": "^3.9.0",
    "typescript": "^3.7.4"
  },
  "jest": {
    "moduleFileExtensions": [
      "js",
      "json",
      "ts"
    ],
    "rootDir": "src",
    "testRegex": ".spec.ts$",
    "transform": {
      "^.+\\.(t|j)s$": "ts-jest"
    },
    "coverageDirectory": "../coverage",
    "testEnvironment": "node"
  }
}

上記のようにpackage.jsonを書き直したら治った。 "main"と"script"の中身が大事。