publish image with github actions

[wozz]

setup GitHub actions to publish the image. if I understand correctly, the backups include the image when it's not published, so by publishing the image, backup size should be reduced

[loganmarchione]

Thanks @wozz! I had struggled with how to do this since I may eventually add new add-ons to this repo. I like your use of --target so that it only targets the prometheus_node_exporter directory.

It was a seamless upgrade for me from 0.8.0 to your 0.8.1 release, thanks!

[wozz]

I probably should have commented a bit more about it...

The main build action is using this marketplace action: https://github.com/marketplace/actions/home-assistant-builder

The other component is the version check - the builder script home-assistant provides doesn't work to check GHCR versions, so I added that part in my own add-on repo and then copied it here. (I wanted to avoid setting up an extra account on docker hub since GitHub already provides a container registry.)

With the version check, the image should only be released when the config file version is changed.

Figured it would be easier to copy my setup for your repo than fork this add-on just for the ~20mb savings in my backups.