Fix tarball structure for podman/docker load compatibility

Remove directory wrapper from OCI tarballs to support podman load and
docker load commands. These tools expect OCI layout files (blobs/,
index.json, oci-layout) at the tarball root, not wrapped in a directory.

Changes:
- Use 'tar -C dir .' instead of 'tar dir' to archive contents without wrapper
- Tarballs now have OCI files at root level for direct loading

Fixes #21

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>
Signed-off-by: Joachim Wiberg <[email protected]>

Author: troglobit

.github/workflows/build-push.yml

@@ -122,7 +122,7 @@ jobs:
               mv ${fn}/rootfs-oci ${name}-oci-${arch}${SFX}
               mv ${fn}/COPYING ${name}-oci-${arch}${SFX}/
               rm -rf -- ${name}-oci-${arch}${SFX}/.[!.]*
-              tar cfz ${name}-oci-${arch}${SFX}.tar.gz ${name}-oci-${arch}${SFX}
+              tar cfz ${name}-oci-${arch}${SFX}.tar.gz -C ${name}-oci-${arch}${SFX} .
           done
           ls -la *-oci*${SFX}/
           for file in *.tar.gz; do