docker execution: trim "image up to date" lines

This line was recently added to docker's output, and is not part of
function execution so should be filtered.

It is also breaking our tests!

internal/fnruntime/container.go

@@ -286,6 +286,7 @@ func isdockerCLIoutput(s string) bool {
 		strings.Contains(s, ": Pull complete") ||
 		strings.Contains(s, "Digest: sha256") ||
 		strings.Contains(s, "Status: Downloaded newer image") ||
+		strings.Contains(s, "Status: Image is up to date for") ||
 		strings.Contains(s, "Unable to find image") {
 		return true
 	}