GOOGLE_APPLICATION_CREDENTIALS should be optional; however, it seems like it isn't #2442

How to reproduce the behaviour

Deploy doccano on GKE or a Google Cloud infrastructure service. Don't provide a value to GOOGLE_APPLICATION_CREDENTIALS.

Your Environment

• Operating System: GKE (Google Cloud Platform Service)
• Used docker image: doccano/docanno

Why is this a bug, and important to have optional

• The infrastructure already has credentials, and when using google cloud apis it uses the default stuff on the machine/infra. Hence providing it is unnecessary and more problematic as you have to set security infrastructure for that.
• Even if we provide the credentials according to the documentation, those credentials will expire and won't persist for a long. Rather it's better to use the default ones on the infra that are refreshed automatically.

Potential fix

You don't need to set credentials in any step and just initialize the client.