Change default value of PIHOLE_DOCKER_TAG in dockerfile

[PromoFaux]

What does this PR aim to accomplish?:

The update checker script from the core repo expects the docker tag to match a specific regex (see: https://regex101.com/r/RsENuz/1)

This prepends the default with dev- so that it is populated correctly:

Also adds a options to build.sh to customise the DOCKER_BUILD_VERSION and PADD_BRANCH build args

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[PromoFaux]

Broke this with a fp :D