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>

tor/Dockerfile

@@ -16,7 +16,7 @@ RUN set -xe \
                           python-dev \
                           tor \
     && curl -sSL https://bootstrap.pypa.io/get-pip.py | python \
-    && pip install fteproxy \
+    && pip install --no-cache-dir fteproxy \
                    obfsproxy \
     && GOPATH=/tmp/obfs4 go get git.torproject.org/pluggable-transports/obfs4.git/obfs4proxy \
         && mv /tmp/obfs4/bin/obfs4proxy /usr/bin \