feat: add automated tagging and pushing of new releases

[bennetgallein]

Adding release process automation.

When master is pushed, release.yml determines the correct version to build (takes LIBRENMS_VERSION from Dockerfile), updated by renovate-bot via PR. Uses the historical commits to determine if an increase in revision (-r1) is required and falls back to "-r0" (this is a change, but technically the initial release wouldn't be a revision IMO. It then uses workflow_dispatch to trigger the usual build-workflow for the new tag (since tags pushed by bot would not trigger a workflow).

[crazy-max]

Thanks for working on this, but don't think we want to automate publishing Docker release tags directly from upstream version bumps.

An upstream LibreNMS release being available is not enough signal that the Docker image is ready to publish. We need to build and test the image first so we can catch upstream regressions or image-specific breakage before pushing a versioned tag. This is the same release policy noted in #209.

A future staging/testing flow could be discussed separately, but this PR bypasses that gate by creating releases from Dockerfile changes. Closing as not planned.