docs: bump go version in credential helper example (#1749)

Author: aioue

docs/private-registries.md

@@ -124,7 +124,7 @@ in a volume that may be mounted onto your watchtower container.
 
 1.  Create the Dockerfile (contents below):
     ```Dockerfile
-    FROM golang:1.16
+    FROM golang:1.17
 
     ENV GO111MODULE off
     ENV CGO_ENABLED 0