The default image name in the code has a bug. It is being set to...

The default image name in the code has a bug. It is being set to packer-{{timestamp}}, the {{timestamp}} part needs to be interpolated. Without the interpolation the GCE builder fails with
the following error

==> googlecompute: Creating image...
==> googlecompute: Error waiting for image: googleapi: Error 400: Invalid value for field 'resource.name': 'packer-{{timestamp}}'. Must be a match of regex '(?:[a-z](?:[-a-z0-9]{0,61}[a-z0-9])?)', invalid

builder/googlecompute/config.go

@@ -73,7 +73,13 @@ func NewConfig(raws ...interface{}) (*Config, []string, error) {
 	}
 
 	if c.ImageName == "" {
-		c.ImageName = "packer-{{timestamp}}"
+		img, err := interpolate.Render("packer-{{timestamp}}", nil)
+		if err != nil {
+			panic(err)
+		}
+
+		// Default to packer-{{ unix timestamp (utc) }}
+		c.ImageName = img
 	}
 
 	if c.InstanceName == "" {