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

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

@@ -28,7 +28,7 @@ RUN apt-get install -y vpcs ubridge
 ADD . /server
 WORKDIR /server
 
-RUN pip3 install -r /server/requirements.txt
+RUN pip3 install --no-cache-dir -r /server/requirements.txt
 
 EXPOSE 3080