feat: allow build secrets args to be passed in cli

[thpiron]

Add possibility to add secrets mount in docker build.

This is mainly to be able to fetch private dependency easily without secrets leaking in the docker image, but it could be useful for other hooks.

[featheredtoast]

👋 hey @thpiron thanks for this - I'm open to the addition of secret args here. Current implementation doesn't use them at all as the initial implementation was translated from the bash script. Curious if it would make sense to pass some of the built in secrets as well, or are these build-time-only secrets (such as ssh keys to private repos) you're passing in?

The runtime secrets are a little complicated as quite often it's expected that those be baked into the final image, so that should continue to be an option.

Can you add some tests here?