Migrate docker image hosting from docker.pkg.github.com to ghcr.io

[Naishy]

Summary

• Move image build/push (ci.yml) and pull (action.yml) from the deprecated docker.pkg.github.com to ghcr.io, using a simplified path (ghcr.io/appvia/githubusermanager).
• Fix the semver tag to keep its v prefix (v1.0.5 instead of 1.0.5) so it matches the tag action.yml actually requests via github.action_ref.
• Update the README's stale docker run ...:main example to ghcr.io/appvia/githubusermanager:latest (a tag that is actually produced by the workflow).

Follow-up required (not part of this PR)

After this merges and the next push/release pushes an image to ghcr.io for the first time, an org admin needs to set the package visibility to public in GitHub package settings — ghcr.io defaults new packages to private, which would otherwise break external consumers pulling the action.

Test plan

• CI (ci.yml) runs successfully on this PR (build/lint/test steps; push step skipped for PRs)
• After merge, confirm a push to main pushes ghcr.io/appvia/githubusermanager:latest and :sha-*
• After next release, confirm ghcr.io/appvia/githubusermanager:vX.Y.Z is pushed and matches what a consumer pinning @vX.Y.Z pulls
• Set package visibility to public, then confirm a consumer workflow can pull and run the action end-to-end