Update private-registries.md

Amazon credential helper updates mean using GO 1.16 will cause slice errors. e.g.

```
undefined: unsafe.Slice
```

Using 1.17 as the source FROM fixes this.

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