Fix Docker ARG variable naming

- Change release-name to release_name (underscores instead of hyphens)
- Update CI workflow to use release_name build arg
- Fixes Docker build error: buildpack-deps:name not found
- Docker now properly resolves to buildpack-deps:unstable

.forgejo/workflows/ci.yml

@@ -62,7 +62,7 @@ jobs:
       - name: Build Docker image
         run: |
           cd /tmp/bootupd-sdboot
-          docker build --build-arg release-name=unstable -f Dockerfile \
+          docker build --build-arg release_name=unstable -f Dockerfile \
             -t bootupd-sdboot:latest .
 
       - name: Build bootupd inside container