Support OCI registry creds

Adds support to 3 environment variables:
- WAREWULF_OCI_USERNAME: registry user username
- WAREWULF_OCI_PASSWORD: registry user password
- WAREWULF_OCI_NOHTTPS: disable tls requirements (for local registries)

Note:
	This will default to ~/.docker/config.json for credentials and other docker
configurations on the system when no options are specified.

internal/pkg/oci/puller.go

@@ -100,7 +100,7 @@ func (p *puller) GenerateID(ctx context.Context, uri string) (string, error) {
 		return "", fmt.Errorf("unable to parse uri: %v", err)
 	}
 
-	src, err := ref.NewImageSource(ctx, nil)
+	src, err := ref.NewImageSource(ctx, p.sysCtx)
 	if err != nil {
 		return "", err
 	}