fix: Use correct key for setting main build context

[cpuguy83]

The signer implementation was not using standard docker keys for setting the main build context, this made it so you can't use signers as standalone frontends executing by docker build.

This is a breaking change as anything implementing a signer will need to be updated to use the correct key.

The main change here is contextKey = "contextkey" instead of contextKey = "context". You can see this in the dockerui package in buildkit: https://github.com/moby/buildkit/blob/d8d946b85c52095d34a52ce210960832f4e06775/frontend/dockerui/context.go#L29

Other changes are just propagating progress groups around so signing is a little easier to track the status of in the build logs and then tests to ensure that the main context is indeed being set as expected.