Add warning about GCP keys

Author: DarthSim

docs/configuration/loading_environment_variables.mdx

@@ -135,6 +135,10 @@ If you run imgproxy inside Google Cloud infrastructure (Compute Engine, Kubernet
 
 Otherwise, set `IMGPROXY_ENV_GCP_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).
 
+:::warning
+For security reasons, imgproxy accepts only service account keys for Google Cloud Secret Manager integration.
+:::
+
 ## Environment file syntax
 
 The following syntax rules apply to environment files:

docs/image_sources/google_cloud_storage.mdx

@@ -32,3 +32,7 @@ gs://%bucket_name/%file_key?generation=%generation
 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 to do 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).
+
+:::warning
+For security reasons, imgproxy accepts only service account keys for Google Cloud Storage integration.
+:::