docs: clarify "ca-certificates" requirement

Fixes #98

docs/google-cloud-storage.md

@@ -8,4 +8,6 @@ You can optionally specify a [storage class](https://cloud.google.com/storage/do
 
 The configured bucket must exist.
 
-This backend is very similar to the [S3](./s3.md) backend, and it has the same limitations
+Google Cloud Storage is exposed over HTTPS so if you are running SFTPGo as docker image please be sure to uncomment the line that install `ca-certificates`, inside your `Dockerfile`, to be able to properly verify certificate authorities.
+
+This backend is very similar to the [S3](./s3.md) backend, and it has the same limitations.

docs/s3.md

@@ -9,6 +9,8 @@ AWS SDK has different options for credentials. [More Detail](https://docs.aws.am
 
 So, you need to provide access keys to activate option 1, or leave them blank to use the other ways to specify credentials.
 
+Most S3 backends require HTTPS connections so if you are running SFTPGo as docker image please be sure to uncomment the line that install `ca-certificates`, inside your `Dockerfile`, to be able to properly verify certificate authorities.
+
 Specifying a different `key_prefix`, you can assign different virtual folders of the same bucket to different users. This is similar to a chroot directory for local filesystem. Each SFTP/SCP user can only access the assigned virtual folder and its contents. The virtual folder identified by `key_prefix` does not need to be pre-created.
 
 SFTPGo uses multipart uploads and parallel downloads for storing and retrieving files from S3.