ServiceException 401 when running gsutil in docker container

The `$GOOGLE_APPLICATION_CREDENTIALS` doesn't seem to work for `gsutil` in a docker container when gcloud is not installed.

For context, I'm working in a docker container which has no `gcloud` installed to minimize storage. The docker requires some files from Google cloud Storage which are copied when the entrypoint is executed.

Description
----------------

I am passing my credentials stored in my env variable `GOOGLE_APPLICATION_CREDENTIALS` during the docker run with:

```
ADC=$GOOGLE_APPLICATION_CREDENTIALS
docker run -v ${ADC}:/tmp/credentials.json:ro -e GOOGLE_APPLICATION_CREDENTIALS=/tmp/credentials.json my_image
```
When running the container, I can see the credentials exist in the by checking the `/tmp` folder. Please note the credentials I'm parsing are exactly the ones as I have locally and work to download via `gsutil`. So far so good.

However, when I run `gsutil cp ` in the container, I get the message:
```
ServiceException: 401 Anonymous caller does not have storage.objects.get access to the Google Cloud Storage object.
```

If I use the google-storage-cloud python option, and in the container I run something like
```python
import google.cloud.storage
google.cloud.storage.Client().download_blob_to_file(sys.argv[1], open(sys.argv[2], 'wb'))"
```
then it works, and the files successfully download from the in GCS. This suggest there is a problem with `gsutil`, not the credentials or authentication via $GOOGLE_APPLICATION_CREDENTIALS.

Expected behavior
---------------------------

I would expect `gsutil` to use the json credentials with no issues, in the same way google cloud storage in Python does.

Start by reproducing the `gsutil cp` command in a minimal Docker container with `GOOGLE_APPLICATION_CREDENTIALS` mounted, then compare its credential handling with the working Python client example. Done means `gsutil` authenticates with the mounted JSON credentials and downloads the requested Cloud Storage object.

Written by the indexing model from the issue text.