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>

youtube/youtube-dl/Dockerfile

@@ -10,7 +10,7 @@ RUN set -xe \
                           ffmpeg \
                           openssl \
                           aria2 \
-    && pip3 install youtube-dl
+    && pip3 install --no-cache-dir youtube-dl
 
 # Try to run it so we know it works
 RUN youtube-dl --version

youtube/youtube-worker/Dockerfile

@@ -7,7 +7,7 @@ MAINTAINER kev <noreplay@easypi.pro>
 
 RUN apk add -U ca-certificates py-pip \
     && rm -rf /var/cache/apk/* \
-    && pip install redis youtube-dl
+    && pip install --no-cache-dir redis youtube-dl
 
 COPY worker.py /code/
 VOLUME /data