diff --git a/README.md b/README.md
index 96ff833..155d499 100644
--- a/README.md
+++ b/README.md
@@ -1,7 +1,8 @@
 # ansible  
 
-Docker images for running and testing ansible playbooks against various operating systems.
-I don't like obfuscated docker images with external scripts and hundreds of bells and whistles.
+Docker images for running and testing ansible playbooks with various operating systems.
+
+I don't like obfuscated Dockerfiles with external scripts and hundreds of bells and whistles, so I created these simple ones.
 
 Available operating systems:
 
@@ -11,7 +12,7 @@ Available operating systems:
 * fedora-24
 * ubuntu-16.04
 
-## run ansible as controller
+## use the ansible image for playbooks
 
     docker run -it --rm \
         -w /site \
@@ -23,21 +24,20 @@ Available operating systems:
 
 Note: /root/.ansible must not be an overlayfs, otherwise ssh accelerate won't work.
 
-## use ansible in drone builds
+## use the ansible image in drone builds
 
     build:
       image: chmuul/ansible:$$OS
-      pull: true
       commands:
         - ansible-playbook -i tests/inventory tests/main.yml --syntax-check
         - ansible-playbook -i tests/inventory tests/main.yml
 
     matrix:
       OS:
-        - ubuntu-16.04
-        - fedora-24
+        - centos-7
         - debian-8
-
+        - fedora-24
+        - ubuntu-16.04
 
 ## references
 
diff --git a/centos-7/Dockerfile b/centos-7/Dockerfile
index 9a0ff12..37615b8 100644
--- a/centos-7/Dockerfile
+++ b/centos-7/Dockerfile
@@ -1,6 +1,8 @@
 FROM centos:7
 
 RUN rpm -iU https://dl.fedoraproject.org/pub/epel/7/x86_64/e/epel-release-7-8.noarch.rpm \
-  && yum -y install ansible
+  && yum -y install ansible \
+  && yum clean headers \
+  && yum clean packages
 
 CMD ["ansible-playbook", "--help"]
diff --git a/debian-8/Dockerfile b/debian-8/Dockerfile
index 632d2a3..0ff6e32 100644
--- a/debian-8/Dockerfile
+++ b/debian-8/Dockerfile
@@ -8,7 +8,7 @@ RUN apt-get -q update \
       python \
       python-dev \
       python-pip \
-    && apt-get autoclean \
+    && apt-get clean \
     && pip install --quiet --upgrade \
       ansible
 
diff --git a/ubuntu-16.04/Dockerfile b/ubuntu-16.04/Dockerfile
index 93398fd..3de378b 100644
--- a/ubuntu-16.04/Dockerfile
+++ b/ubuntu-16.04/Dockerfile
@@ -7,6 +7,6 @@ RUN apt-get update \
     && apt-get update \
     && apt-get install -y --no-install-recommends \
       ansible \
-    && apt-get autoclean
+    && apt-get clean
 
 CMD ["ansible-playbook", "--help"]