diff --git a/playbooks/buildset-registry/pre.yaml b/playbooks/buildset-registry/pre.yaml
index b904358..7c20e32 100644
--- a/playbooks/buildset-registry/pre.yaml
+++ b/playbooks/buildset-registry/pre.yaml
@@ -1,4 +1,7 @@
 - hosts: all
+  vars:
+    # NOTE(yoctozepto): For now, buildset-registry supports only Docker.
+    container_command: docker
   tasks:
     - name: Install docker
       include_role: