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>

urlwatch/Dockerfile

@@ -18,7 +18,7 @@ RUN set -xe \
                           py3-pip          \
                           python3          \
                           python3-dev      \
-    && pip3 install appdirs        \
+    && pip3 install --no-cache-dir appdirs        \
                     cssselect      \
                     keyring        \
                     lxml           \