internal/config: accept GKE versions for reprocessing

Change-Id: Ibe9b3d299d0a86f8ddd5ea20582ca4bdd7357f11
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/257244
Trust: Jonathan Amsterdam <jba@google.com>
Run-TryBot: Jonathan Amsterdam <jba@google.com>
TryBot-Result: kokoro <noreply+kokoro@google.com>
Reviewed-by: Julie Qiu <julie@golang.org>
diff --git a/internal/config/config.go b/internal/config/config.go
index 2eee8f6..c1ed6cf 100644
--- a/internal/config/config.go
+++ b/internal/config/config.go
@@ -75,6 +75,10 @@
 // ValidateAppVersion validates that appVersion follows the expected format
 // defined by AppVersionFormat.
 func ValidateAppVersion(appVersion string) error {
+	// Accept GKE versions, which start with the docker image name.
+	if strings.HasPrefix(appVersion, "gcr.io/") {
+		return nil
+	}
 	if _, err := time.Parse(AppVersionFormat, appVersion); err != nil {
 		// Accept alternative version, used by our AppEngine deployment script.
 		const altDateFormat = "2006-01-02t15-04"