Add Docker metadata action for image tagging & labeling

[stumpylog]

This PR integrates docker/metadata-action to automatically generate Docker image tags and labels following best practices. Although we could use only the labels, I also updated to use the action for outputting the tags information as well, for consistency in setting them in one location. In particular, this could allow disabling of DockerHub tags for forks (which would make it easier to test and contribute change to the CI flow).

Using the metadata action brings in standard OCI labels for information such as image URL, Git revisions, build date & time, description and license. And probably more, all for free.

Comparing the last tags used in the release of 0.0.42, the produced tags are the same, with the addition of:

--label org.opencontainers.image.created=2025-10-29T15:06:58.363Z
--label org.opencontainers.image.description=Open Source Uptime Kuma Alternative
--label org.opencontainers.image.licenses=MIT
--label org.opencontainers.image.revision=29458d423802c5497c12ad18cde59b80d9c9bae3
--label org.opencontainers.image.source=https://github.com/stumpylog/peekaping
--label org.opencontainers.image.title=peekaping
--label org.opencontainers.image.url=https://github.com/stumpylog/peekaping
--label org.opencontainers.image.version=0.0.42-amd64

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