azure: Specify different Azure storage in the shell env (#10943)

AZURE_STORAGE_ACCOUNT and AZURE_STORAGE_KEY are used in 
azure CLI to specify the azure blob storage access & secret keys. With this commit, 
it is possible to set them if you want the gateway's own credentials to be
different from the Azure blob credentials.

Co-authored-by: Harshavardhana <harsha@minio.io>

cmd/gateway/azure/gateway-azure.go

@@ -173,6 +173,16 @@ func (g *Azure) Name() string {
 
 // NewGatewayLayer initializes azure blob storage client and returns AzureObjects.
 func (g *Azure) NewGatewayLayer(creds auth.Credentials) (minio.ObjectLayer, error) {
+	var err error
+
+	// Override credentials from the Azure storage environment variables if specified
+	if acc, key := env.Get("AZURE_STORAGE_ACCOUNT", creds.AccessKey), env.Get("AZURE_STORAGE_KEY", creds.SecretKey); acc != "" && key != "" {
+		creds, err = auth.CreateCredentials(acc, key)
+		if err != nil {
+			return nil, err
+		}
+	}
+
 	endpointURL, err := parseStorageEndpoint(g.host, creds.AccessKey)
 	if err != nil {
 		return nil, err

docs/gateway/azure.md

@@ -36,6 +36,10 @@ mc ls myazure
 [2017-02-26 22:10:11 PST]     0B test-container1/
 ```
 
+### Use custom access/secret keys
+
+If you do not want to share the credentials of the Azure blob storage with your users/applications, you can set the original credentials in the shell environment using `AZURE_STORAGE_ACCOUNT` and `AZURE_STORAGE_KEY` variables and assign different access/secret keys to `MINIO_ACCESS_KEY` and `MINIO_SECRET_KEY`.
+
 ### Known limitations
 Gateway inherits the following Azure limitations: