chore: Use --no-cache-dir flag to pip in Dockerfiles, to save space

Using "--no-cache-dir" flag in pip install ,make sure dowloaded packages
by pip don't cached on system . This is a best practise which make sure
to fetch ftom repo instead of using local cached one . Further , in case
of Docker Containers , by restricing 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>

shadowsocks/arm/Dockerfile

@@ -8,7 +8,7 @@ MAINTAINER kev <noreply@easypi.pro>
 RUN set -ex \
     && apk add -U curl libsodium python \
     && curl -sSL https://bootstrap.pypa.io/get-pip.py | python \
-    && pip install shadowsocks \
+    && pip install --no-cache-dir shadowsocks \
     && apk del curl \
     && rm -rf /var/cache/apk/*