set DEBIAN_FRONTEND environment variable when using apt
diff --git a/containers/xos/Dockerfile b/containers/xos/Dockerfile
index 2499187..d8fc4ba 100644
--- a/containers/xos/Dockerfile
+++ b/containers/xos/Dockerfile
@@ -8,7 +8,7 @@
 # XXX End workaround
 
 # Install.
-RUN apt-get update && apt-get install -y \
+RUN DEBIAN_FRONTEND=noninteractive apt-get update && apt-get install -y \
     curl \
     gcc \
     geoip-database \