@RichiH - automate Docker Hub image creation

[openenergyprojects]

We are trying to push a helm chart for this nice project of yours (prometheus-community/helm-charts#3417)
For that to be completed, we require up to date images for your project (current one is 2+ years old).
We would appreciate if you would do the 2 steps mentioned by @DaAwesomeP:

          @RichiH about the old Docker Hub images, if you provide Docker Hub secrets then the CI will already publish: https://github.com/RichiH/modbus_exporter/blob/a144551d67136fc7c17eb8ad9146b3edb0f68821/.github/workflows/ci.yml#L107-L110

Originally posted by @DaAwesomeP in #57 (comment)