Use buildx to create docker test images

[yubiuser]

What does this PR aim to accomplish?:

We use docker to build different images to run our tests on. So far, we use docker build to create the images. However, this command is considered deprecated and newer docker instances will warn

DEPRECATED: The legacy builder is deprecated and will be removed in a future release.
            Install the buildx component to build images with BuildKit:
            https://docs.docker.com/go/buildx/

This PR will use the recommended docker buildx build command.

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[pralor-bot]

This pull request has been mentioned on Pi-hole Userspace. There might be relevant details there:

https://discourse.pi-hole.net/t/pi-hole-ftl-v5-22-web-v5-19-and-core-v5-16-1-released/61999/1