Expose more UI feature flags inside Dockerfile

[ShantanuKumar]

Summary

Export more feature flags defined in the
https://github.com/Netflix/metaflow-ui/blob/master/docs/feature-flags.md

Motivation

We would like to use some of these feature flags.

Describe alternatives you've considered

The only alternative would be to build our own docker image which we would like to avoid

Additional context

[obgibson]

Do you have any particular flags that you want exposed? How do you plan to set them?