Allow false env var value

[junket]

I am booting an accessory that needs a clear env var value like ENABLE_AUTO_LOGIN=false so my config is just:

env:
  clear:
    ENABLE_AUTO_LOGIN: false

When Kamal passes such env vars to docker run it drops the false value since the argumentize utility checks that the value is present? and false values evaluate to false, leading to docker run --env ENABLE_AUTO_LOGIN

This PR updates argumentize to preserve false values, leading to docker run --env ENABLE_AUTO_LOGIN=false