Add BuildKit definition file for cross-platform registry image

[paullj1]

Just run:

docker builds bake -f docker-bake.hcl builder
docker builds bake -f docker-bake.hcl caddy

Buildkit will then build and push a multi-image registry manifest so it won't matter which architecture the client is using, they'll get the correct image (ARMv6, ARMv7, ARM64, or amd64).

[paullj1]

FWIW, I built and pushed the latest using this to: paullj1/caddy:latest, and paullj1/caddy:builder