GCS - No documentation for non-gke authentication

I have noticed on the helm chart for chartmuseum (`stable/chartmuseum`) that there is no way to pass the service account private file (`.json` file generated when creating the Storage Account created on Google Cloud.

What I am trying to do is to deploy chartmuseum, using the helm chart, and connecting it to a bucket on GCS, in a cluster that is not living on GKE. **The documentation** for both the chart and the chartmuseum **does not provide any link or guide to setup Chartmuseum with GCS without GKE** (that provides the credentials by default).

What I was able to achieve is to set up the environment variable `GOOGLE_APPLICATION_CREDENTIALS` in `env.secret`, but there is no other way to provide the file/mount an extra volume.

Here is the logs after deploying the helm chart:

```
W0409 18:04:32.223131 29387 cmd.go:353] log is DEPRECATED and will be removed in a future version. Use logs instead.
panic: dialing: google: error getting credentials using GOOGLE_APPLICATION_CREDENTIALS environment variable: open /data/hello-world.json: no such file or directory

goroutine 1 [running]:
github.com/kubernetes-helm/chartmuseum/pkg/storage.NewGoogleCSBackend(0xc420016016, 0x13, 0x0, 0x0, 0x0)
/go/src/github.com/kubernetes-helm/chartmuseum/pkg/storage/google.go:24 +0x36d
main.googleBackendFromContext(0xc4201b4dc0, 0x6, 0xf38c5a)
/go/src/github.com/kubernetes-helm/chartmuseum/cmd/chartmuseum/main.go:120 +0xe9
main.backendFromContext(0xc4201b4dc0, 0xd81f60, 0xc4202ce8e0)
/go/src/github.com/kubernetes-helm/chartmuseum/cmd/chartmuseum/main.go:81 +0x3f9
main.cliHandler(0xc4201b4dc0)
/go/src/github.com/kubernetes-helm/chartmuseum/cmd/chartmuseum/main.go:38 +0x43
github.com/kubernetes-helm/chartmuseum/vendor/github.com/urfave/cli.HandleAction(0xd7e6e0, 0xf6d800, 0xc4201b4dc0, 0xc4201b90e0, 0x0)
/go/src/github.com/kubernetes-helm/chartmuseum/vendor/github.com/urfave/cli/app.go:492 +0x7c
github.com/kubernetes-helm/chartmuseum/vendor/github.com/urfave/cli.(*App).Run(0xc420214b60, 0xc42000a1a0, 0x2, 0x2, 0x0, 0x0)
/go/src/github.com/kubernetes-helm/chartmuseum/vendor/github.com/urfave/cli/app.go:264 +0x6ac
main.main()
/go/src/github.com/kubernetes-helm/chartmuseum/cmd/chartmuseum/main.go:34 +0x1ef
```