chore : use --no-cache-dir flag to pip in dockerfiles to save space (#7898)

using --no-cache-dir flag in pip install ,make sure downloaded packages
by pip don't cached on system . This is a best practice which make sure
to fetch from repo instead of using local cached one . Further , in case
of Docker Containers , by restricting caching , we can reduce image size.
In term of stats , it depends upon the number of python packages
multiplied by their respective size . e.g for heavy packages with a lot
of dependencies it reduce a lot by don't caching pip packages.

Further , more detail information can be found at

https://medium.com/sciforce/strategies-of-docker-images-optimization-2ca9cc5719b6

Signed-off-by: Pratik Raj <rajpratik71@gmail.com>

Dockerfile

@@ -16,9 +16,9 @@ RUN curl -fsSL https://download.docker.com/linux/ubuntu/gpg | apt-key add - \
 
 WORKDIR /kubespray
 COPY . .
-RUN /usr/bin/python3 -m pip install pip -U \
-    && /usr/bin/python3 -m pip install -r tests/requirements.txt \
-    && python3 -m pip install -r requirements.txt \
+RUN /usr/bin/python3 -m pip install --no-cache-dir pip -U \
+    && /usr/bin/python3 -m pip install --no-cache-dir -r tests/requirements.txt \
+    && python3 -m pip install --no-cache-dir -r requirements.txt \
     && update-alternatives --install /usr/bin/python python /usr/bin/python3 1
 
 RUN KUBE_VERSION=$(sed -n 's/^kube_version: //p' roles/kubespray-defaults/defaults/main.yaml) \