diff --git a/Dockerfile b/Dockerfile
index 496cd16..e98b537 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -8,6 +8,8 @@ ENV MIRROR_CATEGORIES main,universe,restricted,multiverse
 ENV UBUNTU_MIRROR_SOURCE archive.ubuntu.com
 ENV UBUNTU_MIRROR_PROJECTS bionic,bionic-updates,bionic-security,bionic-backports,focal,focal-updates,focal-security,focal-backports
 
+ENV THIS_MIRROR_URL http://172.17.0.1/ubuntu/
+
 ADD ./sources.list /etc/apt/sources.list
 RUN echo "#!/bin/sh\nexit 101" > /usr/sbin/policy-rc.d
 # phusion baseimage says i'm not supposed to do this