The hook is run before the environment is loaded or the config is
parsed.
This makes it a bit of a special case - it doesn't have the usual
KAMAL_XYZ environment variables, as we haven't parsed the config.
The use case for this is to do auth checking or setup. So for example
we can confirm you are logged in to a secret manager, and then you
can directly call it to load your secrets in the .kamal/.env file
using .dotenv's
[command substitution](https://github.com/bkeepers/dotenv?tab=readme-ov-file#command-substitution).
Validate the Kamal configuration giving useful warning on errors.
Each section of the configuration has its own config class and a YAML
file containing documented example configuration.
You can run `kamal docs` to see the example configuration, and
`kamal docs <section>` to see the example configuration for a specific
section.
The validation matches the configuration to the example configuration
checking that there are no unknown keys and that the values are of
matching types.
Where there is more complex validation - e.g for envs and servers, we
have custom validators that implement those rules.
Additonally the configuration examples are used to generate the
configuration documentation in the kamal-site repo.
You generate them by running:
```
bundle exec bin/docs <kamal-site-checkout>
```
