Google App Engine Go 1.11 Runtime Doesn't Cache Dependencies

The documentation of Google App Engine mentions that the Go 1.11 runtime, in the standard environment, caches fetched dependencies to reduce build times.

But it looks like the cache mentioned in the documentation is not used when using Go modules (with a go.mod file).

This repository is used to reproduce the issue following this methodology:

1. Host the source code of a dummy dependency at github.com/ngrilly/test-golang-cache/hello. The import path of the dependency is test-golang-cache.appspot.com/hello.
2. Host a static web page containing a go-import meta tag referencing the GitHub repository at test-golang-cache.appspot.com/hello.
3. Deploy to App Engine a dummy web server that depends on test-golang-cache.appspot.com/hello.
4. Check the log of requests served by test-golang-cache.appspot.com to verify if the dependency was fetched or not.
5. Repeat steps 3 and 4 several times to check that the cache works or not.

Steps to reproduce

Checkout the projet outside of your $GOPATH (working outside your $GOPATH is important to enable Go modules):

$ git clone https://github.com/ngrilly/test-golang-cache.git

Set your default project:

$ gcloud config set project test-golang-cache

Deploy the static website containing the go-import meta tag:

$ gcloud app deploy static

Make sure the server runs locally:

$ cd server
$ go run .

Deploy to Google App Engine:

$ gcloud app deploy

In the Google Cloud console, go to Cloud Build > History, open the last build, and check the logs. You should see something similar to this:

...
Step #1 - "builder": 2019/05/02 15:23:01 Running &{/usr/local/go/bin/go [go build -o /tmp/staging273166152/usr/local/bin/start .] [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=fc04630b97f5 HOME=/builder/home BUILDER_OUTPUT=/builder/outputs DEBIAN_FRONTEND=noninteractive GOROOT=/usr/local/go/ GOPATH=/go GO111MODULE=on GOCACHE=/tmp/cache103838215 GOPATH=/go] /tmp/staging273166152/srv <nil> <nil> <nil> [] <nil> <nil> <nil> <nil> <nil> false [] [] [] [] <nil> <nil>}
Step #1 - "builder": 2019/05/02 15:24:13 Wrote build output to /builder/outputs/output
...

In this case, go build runs for 1 minute and 12 seconds (it starts at 15:23:01 and ends at 15:24:13).

Now go to Logging > Logs and check if there is a request for /hello?go-get=1.

Results

Each time I deploy with gcloud app deploy, there is an HTTPS request fetching the dependency at test-golang-cache.appspot.com/hello. It looks like the cache mentioned in the documentation is not used.