fix(ci): rename GITHUB_TOKEN to GHCR_TOKEN for Forgejo compat

Forgejo reserves GITHUB_TOKEN as a protected secret name.
Use GHCR_USERNAME + GHCR_TOKEN instead.

.forgejo/workflows/docker.yml

@@ -38,8 +38,8 @@ jobs:
         uses: https://github.com/docker/login-action@v3
         with:
           registry: ghcr.io
-          username: ${{ github.repository_owner }}
-          password: ${{ secrets.GITHUB_TOKEN }}
+          username: ${{ secrets.GHCR_USERNAME }}
+          password: ${{ secrets.GHCR_TOKEN }}
 
       - name: Login to Docker Hub
         if: github.event_name != 'pull_request'