--- a/dockerfiles/ubuntu-xenial	Sun Jul 24 09:33:01 2016 +0800
+++ b/dockerfiles/ubuntu-xenial	Mon Jul 25 14:03:08 2016 +0800
@@ -2,13 +2,14 @@
 
 RUN echo 'APT::Install-Recommends "no";' > /etc/apt/apt.conf.d/99recommends
 RUN apt-get update && DEBIAN_FRONTEND=noninteractive apt-get install -y \
-  build-essential \
-  git \
-  mercurial \
-  python-yaml \
-  luarocks \
-  unzip \
-  npm \
-  virtualenv
+    build-essential \
+    git \
+    mercurial \
+    python-yaml \
+    luarocks \
+    unzip \
+    npm \
+    virtualenv \
+ && rm -rf /var/lib/apt/lists/*
 
 RUN useradd worker --create-home --user-group