build(aur): add docker-buildx dependency

[stasadev]

The Issue

• chore(deps): bump docker-compose to v2.39.2, remove COMPOSE_BAKE=false #7545

DDEV v1.24.8 uses docker-compose v2.39.3, which requires docker-buildx https://archlinux.org/packages/extra/x86_64/docker-buildx/

(docker-compose v2.37.0 switched to Bake builder by default.)

How This PR Solves The Issue

Adds missing AUR dependency.

Manual Testing Instructions

Automated Testing Overview

Release/Deployment Notes

[rfay]

Good idea. Not sure why the docker package doesn't already have a dependency on it though. Regardless, this does no harm.

[stasadev]

Not sure why the docker package doesn't already have a dependency on it though

docker-buildx is an optional dependency for docker.

In fact, this should be required by docker-compose (but it won't have any effect on ddev-bin, since we don't require docker-compose).