ci: add dependabot

[resmo]

Ensure we update github actions and don't miss image releases (I am not sure if it will work they way we use args in dockerfile for the version.

[znerol]

Thanks. According to the dependabot docker docs we also need to add the following line to the Dockerfile:

LABEL org.opencontainers.image.source="https://github.com/prometheus-pve/prometheus-pve-exporter/"

The docs also mentions that tags need to be in the same format on the git repo as well as in the image registry. Unfortunately we are using vX.Y.Z on the repo and X.Y.Z in the image registry. Guess we have to decide whether we change one or the other.

Regarding the base image version arg in the docker image. I'm open to drop those bits. I cannot imagine a use case where supplying a different base image version via build args would simplify anything.

[resmo]

I think there is a misunderstanding: the images we depend on must fulfill the requirements you described. Our image doesn't have to.

[znerol]

Ok, let's see what happens.