docs: clarify s3 build feature requirements

.env.template

@@ -36,6 +36,10 @@
 ## Credentials in URI query params are supported as a last resort, but it is
 ## strongly recommended to use environment credentials/IAM instead.
 ##
+## Note: For S3 paths to work, the container/binary must be built with both
+## a DB backend and the `s3` feature (for example: `sqlite,s3`,
+## `postgresql,s3`, or `mysql,s3`).
+##
 ## When using an external location, make sure to set TMP_FOLDER,
 ## TEMPLATES_FOLDER, and DATABASE_URL to local paths and/or a remote database
 ## location.

README.md

@@ -123,6 +123,7 @@ s3://bucket/prefix?endpoint=https%3A%2F%2Fs3.example.internal&enable_virtual_hos
 - MinIO/Ceph usually require `endpoint` and `enable_virtual_host_style=false`.
 - Cloudflare R2 usually requires `endpoint` and often `region=auto`.
 - To omit `x-amz-storage-class`, set `default_storage_class=` (empty value).
+- Container images must include both a DB backend feature and `s3` (for example `sqlite,s3`, `postgresql,s3`, or `mysql,s3`).
 
 Kubernetes example: