I'm trying to modify some code snippets of flytekit and test a workflow against the customized version, how could I test the remote execution without building a new docker image?

You could use fast registration. Have you tried running the

pyflyte run …

command after you modified your code?

Yes, I have tried locally run with the command above.

For just code changes, you need not build a Docker image, and if flytekit along with a few other Python packages like numpy and pandas are the only dependencies you need, then you need not specify any image to the pyflyte run command. But if you need additional dependencies, you’ll have to build one. In a nutshell, use the default image or build the image once, and use the same image to register your modified code again. We also have a guide you can refer to, to build large apps: https://docs.flyte.org/projects/cookbook/en/latest/auto/larger_apps/index.html. The fast registration-related instructions are present here.