Check --cache-repo is provided with --cache and --no-push

As described in #373, kaniko panics when provided with --cache and --no-push since it tries to infer a cache repo from the destination, which doesn't exist.

To fix this, I added a check to make sure --cache-repo is passed in when both these flags are provided.

cmd/executor/cmd/root.go

@@ -55,6 +55,9 @@ var RootCmd = &cobra.Command{
 		if !opts.NoPush && len(opts.Destinations) == 0 {
 			return errors.New("You must provide --destination, or use --no-push")
 		}
+		if err := cacheFlagsValid(); err != nil {
+			return errors.Wrap(err, "cache flags invalid")
+		}
 		if err := resolveSourceContext(); err != nil {
 			return errors.Wrap(err, "error resolving source context")
 		}
@@ -112,6 +115,19 @@ func checkContained() bool {
 	return err == nil
 }
 
+// cacheFlagsValid makes sure the flags passed in related to caching are valid
+func cacheFlagsValid() error {
+	if !opts.Cache {
+		return nil
+	}
+	// If --cache=true and --no-push=true, then cache repo must be provided
+	// since cache can't be inferred from destination
+	if opts.CacheRepo == "" && opts.NoPush {
+		return errors.New("if using cache with --no-push, specify cache repo with --cache-repo")
+	}
+	return nil
+}
+
 // resolveDockerfilePath resolves the Dockerfile path to an absolute path
 func resolveDockerfilePath() error {
 	if util.FilepathExists(opts.DockerfilePath) {