Add a `compose build --print` flag

[emersion]

Description

docker buildx bake has a convenient --print flag to print the final bake file as JSON. docker buildx supports compose files, however it lags behind on some features (e.g. services in additional_contexts). It would be nice to add the same --print flag to compose.

[ndeloof]

If you run COMPOSE_BAKE=true docker compose --verbose build you'll set in logs the bake file being produced by compose to run the build

[emersion]

Yeah, but I'd like to save this to a file and feed this to another tool. I can do that with docker buildx bake --print >bake.json, but not reliably with docker-compose.

[emersion]

Thank you so much!

(Note, I would've been willing to send a PR - next time feel free to ask me to do so :) )