diff --git a/docker/backend/Dockerfile b/docker/backend/Dockerfile
index 160623630..3d9c19eed 100644
--- a/docker/backend/Dockerfile
+++ b/docker/backend/Dockerfile
@@ -1,5 +1,8 @@
 FROM docker.io/ubuntu:latest
 
+# DEBUG: list apt sources
+RUN grep ^ /etc/apt/sources.list /etc/apt/sources.list.d/*
+
 # compile-time dependencies
 RUN apt-get -y update && apt-get -y install inotify-tools
 RUN apt-get -y update && apt-get -y install haskell-stack git