Secrets injection

[pedroct92]

First of all, thanks a lot for the repo it helped me a lot on implementing kamal.

I would like to point out a small improvement (or keep it here as a reference) with secrets being injected. The current build creates a .env on build time, however, that would produce docker images being tagged with "uncommitted" changes tag. So to avoid that, we can pass secrets on a more "kamal way".

env:
 clear:
   NEXT_PUBLIC_TOTO: 123abc
 secret:
   - BASE_URL 

So public params can be added directly on the deployment file and secret ones will be read from .kamal/secrets. The secrets will be read from env variables $BASE_URL injected from GH.

Thanks.