added autobuild and autorelease workflow for the newest build

[liblame]

Took the pr 147 from @tritethunder as an idea and reworked it a little bit. Sorry if i could have done that in his pr i am pretty new to git and github.

The workflow now checks if there is a new pihole container release, builds it and creates a new release in this repo.

Only caveat is that actions/create-release is not maintained anymore and uses NodeJS 12 and set-output. So maybe creating it automatically is not the best path.

You still have to put your dockerhub username and token under settings->secrets->actions

[watchingdogs]

@aleksanderbl29 How come then that this is not yet merged?

[aleksanderbl29]

@watchingdogs I can't seem to find a good reason other than the fact that chriscrowe hasn't been active for some time in this repo. Personally, I've forked the repo and implemented this workflow so I can have the newest updates.

On the other hand you could question if it is really needed to have the absolute newest updates at all times - I like it, but def isn't needed.