无法在 App Engine 标准环境中部署 go1.11：应用程序不在 GOPATH 中

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我尝试将我的 go 应用部署到 app engine。我有以下构建错误：

starting step #1 - "builder"
step #1 - "builder": pulling image: gcr.io/gae-runtimes/go111_app_builder:go111_1_11_2_20181111_rc00
step #1 - "builder": go111_1_11_2_20181111_rc00: pulling from gae-runtimes/go111_app_builder
step #1 - "builder": digest: sha256:51fb36bfa16e7013356867c3a3972986084df93e56258fc258579a5799f0436e
step #1 - "builder": status: downloaded newer image for gcr.io/gae-runtimes/go111_app_builder:go111_1_11_2_20181111_rc00
step #1 - "builder": 2018/11/24 18:13:29 your app is not on your gopath, this build may fail.
step #1 - "builder": 2018/11/24 18:13:29 building from go source in /tmp/staging477638319/srv, with main package at ./...
step #1 - "builder": 2018/11/24 18:13:29 building /tmp/staging477638319/srv, saving to /tmp/staging477638319/usr/local/bin/start
step #1 - "builder": 2018/11/24 18:13:30 wrote build output to /builder/outputs/output
step #1 - "builder": 2018/11/24 18:13:30 failed to build app: your app is not on your gopath, please move it there and try again.
step #1 - "builder": building app with command '[go build -o /tmp/staging477638319/usr/local/bin/start ./...]', env '[path=/go/bin:/usr/local/go/bin:/builder/google-cloud-sdk/bin/:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin hostname=d253e517b16c home=/builder/home builder_output=/builder/outputs debian_frontend=noninteractive goroot=/usr/local/go/ gopath=/go gopath=/tmp/staging477638319/srv/gopath]': err=exit status 1, out=srv/main.go:7:2: cannot find package "cloud.google.com/go/firestore" in any of:
step #1 - "builder": /usr/local/go/src/cloud.google.com/go/firestore (from $goroot)
step #1 - "builder": /tmp/staging477638319/srv/gopath/src/cloud.google.com/go/firestore (from $gopath)
step #1 - "builder": srv/main.go:8:2: cannot find package "github.com/gin-gonic/gin" in any of:
step #1 - "builder": /usr/local/go/src/github.com/gin-gonic/gin (from $goroot)
step #1 - "builder": /tmp/staging477638319/srv/gopath/src/github.com/gin-gonic/gin (from $gopath)
step #1 - "builder": srv/main.go:9:2: cannot find package "google.golang.org/api/option" in any of:
step #1 - "builder": /usr/local/go/src/google.golang.org/api/option (from $goroot)
step #1 - "builder": /tmp/staging477638319/srv/gopath/src/google.golang.org/api/option (from $gopath)
finished step #1 - "builder"
error
error: build step 1 "gcr.io/gae-runtimes/go111_app_builder:go111_1_11_2_20181111_rc00" failed: exit status 1

我的 app.yaml 文件如下所示：

runtime: go111
handlers:
- url: /api/user
  script: auto

- url: /favicon.ico
  static_files: build/favicon.ico
  upload: build/favicon.ico

- url: /
  static_files: build/index.html
  upload: build/index.html

- url: /
  static_dir: build

根目录下的 main.go 文件如下所示：

package main

import (
    "context"
    "net/http"

    "cloud.google.com/go/firestore"
    "github.com/gin-gonic/gin"
    "google.golang.org/api/option"
)

const firestoreAccountFile = "firebase.json"
const firestoreProjectID = "golang-gae-firestore-template"

type formData struct {
    Name  string `json:"name" binding:"required"`
    Email string `json:"email" binding:"required"`
}

func main() {
    // Gin init
    r := gin.Default()
    // Serve from static build directory
    r.StaticFS("/", http.Dir("./build"))
    // routes
    r.POST("/api/user", userHandler)
    // run application on port 8080
    r.Run(":8080")
}

func writeLogIfError(c *gin.Context, err error) {
    if err != nil {
        c.JSON(http.StatusBadRequest, gin.H{"error": err.Error()})
    }
}

func getNewFirestoreClient(ctx context.Context) (*firestore.Client, error) {
    return firestore.NewClient(ctx, firestoreProjectID, option.WithServiceAccountFile(firestoreAccountFile))
}

func userHandler(c *gin.Context) {
    ctx := context.Background()

    client, err := getNewFirestoreClient(ctx)
    writeLogIfError(c, err)
    defer client.Close()
    // Get form data
    var form formData
    c.BindJSON(&form)

    // [START add user to firestore]
    _, _, err = client.Collection("users").Add(ctx, map[string]interface{}{
        "name":  form.Name,
        "email": form.Email,
    })
    if err != nil {
        c.JSON(http.StatusBadRequest, gin.H{"error": err.Error()})
        return
    }
    // [END add user to firestore]
    c.JSON(http.StatusOK, gin.H{"status": "user added to db"})
}

如果没有出现这些构建错误，我就无法成功构建。我尝试完全遵循 app engine 上的 go 文档，但如果我要构建的应用程序与建议的不同，我会感到困惑。关于如何解决此 gopath 错误有什么想法吗？

解决方案

更新：我无法使用 GOPATH 成功部署，而是在包含环境变量后能够使用 go.mod 成功部署：export GO111MODULE=on 以使模块正常工作。此处的文档：https://github.com/golang/go/wiki/Modules。

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