Fail on missing scheme in the configured Issuer URL

server/server.go

@@ -173,6 +173,8 @@ func newServer(ctx context.Context, c Config, rotationStrategy rotationStrategy)
 	issuerURL, err := url.Parse(c.Issuer)
 	if err != nil {
 		return nil, fmt.Errorf("server: can't parse issuer URL")
+	} else if issuerURL.Scheme == "" {
+		return nil, fmt.Errorf("server: configured issuer URL doesn't include http(s):// scheme")
 	}
 
 	if c.Storage == nil {