diff --git a/Dockerfile b/Dockerfile
index 7da658219..60ba4ff57 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -23,10 +23,11 @@ RUN set -x \
 RUN set -x \
     && add-apt-repository ppa:george-edison55/cmake-3.x
 
+ENV QT_PPA=qt591
 ENV QT_VERSION=qt59
 
 RUN set -x \
-    && add-apt-repository --yes ppa:beineri/opt-${QT_VERSION}-trusty
+    && add-apt-repository --yes ppa:beineri/opt-${QT_PPA}-trusty
 
 
 RUN set -x \