code: public RestTemplateBuilder rootUri(String rootUri) {
  		return new RestTemplateBuilder(this.detectRequestFactory, rootUri,
  				this.messageConverters, this.requestFactorySupplier,
  				this.uriTemplateHandler, this.errorHandler, this.basicAuthentication,
  				this.restTemplateCustomizers, this.requestFactoryCustomizer,
  				this.interceptors);
  	}
docs: Set a root URL that should be applied to each request that starts with {@code '/'}.
  See {@link RootUriTemplateHandler} for details.
  @param rootUri the root URI or {@code null}
  @return a new builder instance