re-add the PIHOLE_DOCKER_TAG build arg that went missing along the way

[PromoFaux]

What does this PR aim to accomplish?:

While we still use pihole -v and pihole updatechecker (these may be removed from the container at a later date), reimplement the same behaviour as the v5 containers - in which the PIHOLE_DOCKER_TAG is baked into the image and stored at /pihole.docker.tag per:

docker-pi-hole/src/Dockerfile

Line 5 in 958f401

RUN echo "${PIHOLE_DOCKER_TAG}" > /pihole.docker.tag

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[sonarcloud]

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