If you run imgproxy inside Google Cloud infrastructure (Compute Engine, Kubernetes Engine, App Engine, and Cloud Functions, etc), and you have granted access to your bucket to the service account, you probably don't need doing anything here. imgproxy will try to use the credentials provided by Google.
Otherwise, set `IMGPROXY_GCS_KEY` environment variable to the content of Google Cloud JSON key. Get more info about JSON keys: [https://cloud.google.com/iam/docs/creating-managing-service-account-keys](https://cloud.google.com/iam/docs/creating-managing-service-account-keys).