use --no-cache-dir flag to pip in dockerfiles to save space

Dockerfile.rke

@@ -14,4 +14,4 @@ RUN wget https://github.com/rancher/rke/releases/download/$RKE_VERSION/rke_linux
     mv kubectl /bin/kubectl && \
     chmod +x /bin/kubectl  && \
     cd $WORKSPACE && \
-    pip install -r requirements.txt
+    pip install --no-cache-dir -r requirements.txt

Dockerfile.v3api

@@ -15,4 +15,4 @@ RUN wget https://storage.googleapis.com/kubernetes-release/release/$KUBECTL_VERS
     mv rke_linux-amd64 /bin/rke && \
     chmod +x /bin/rke && \
     cd $WORKSPACE && \
-    pip install -r requirements_v3api.txt
+    pip install --no-cache-dir -r requirements_v3api.txt

images/container-utils/Dockerfile

@@ -7,6 +7,6 @@ RUN apt-get update -y && \
     apt-get install -y python-pip python-dev build-essential curl dnsutils iputils-ping openssh-server net-tools && \
     mkdir /var/run/sshd && echo 'root:screencast' | chpasswd &&  \
     sed -i 's/PermitRootLogin prohibit-password/PermitRootLogin yes/' /etc/ssh/sshd_config && \
-    pip install -r requirements.txt
+    pip install --no-cache-dir -r requirements.txt
 
 CMD ["/app/start.sh"]