diff --git a/Dockerfile b/Dockerfile
index cec95d4..8c80523 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -42,4 +42,9 @@ RUN dbus-uuidgen --ensure=/etc/machine-id
 ENV PLAN9=/usr/lib/plan9
 ENV PATH=$PATH:/usr/lib/plan9/bin
 
+# 9c and 9l filter their own output through $egrep, which plan9port never
+# assigns, so the pipeline dies and takes every warning and error with it.
+# Both source $PLAN9/config, which the distribution does not ship.
+RUN echo 'egrep="grep -E"' > $PLAN9/config
+
 WORKDIR /src