Bump sigstore/cosign-installer from 2.7.0 to 2.8.0 (#31)

Bumps [sigstore/cosign-installer](https://github.com/sigstore/cosign-installer) from 2.7.0 to 2.8.0.
- [Release notes](https://github.com/sigstore/cosign-installer/releases)
- [Commits](sigstore/cosign-installer@v2.7.0...v2.8.0)

---
updated-dependencies:
- dependency-name: sigstore/cosign-installer
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

.github/workflows/docker-publish.yml

@@ -58,7 +58,7 @@ jobs:
       # https://github.com/sigstore/cosign-installer
       - name: Install cosign
         if: github.event_name == 'release'
-        uses: sigstore/cosign-installer@v2.7.0
+        uses: sigstore/cosign-installer@v2.8.0
         with:
           cosign-release: 'v1.9.0'