Running pnpm build fails

[guidanoli]

Ever since we've switched to a more target platform-agnostic approach to our Dockerfile, the pnpm build command (which, in turn, runs cartesi build) has been failing. That's because the Cartesi CLI doesn't specify the target platform when it runs docker buildx build under-the-hood. This is currently being tracked by cartesi/cli#73.

In the meantime, we need to run docker build and cartesi build --from-image separately. This can be achieved with a Makefile target which runs both. Progress can be tracked by an image ID file (see --iidfile option of docker build) and the image.ext2 file under .cartesi.