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>

slacker/Dockerfile

@@ -7,7 +7,7 @@ MAINTAINER EasyPi Software Foundation
 
 RUN set -xe \
  && apk add --no-cache curl \
- && pip install aiosmtpd atpublic pyslack pyyaml requests \
+ && pip install --no-cache-dir aiosmtpd atpublic pyslack pyyaml requests \
  && cd /usr/local/lib/python3.9/site-packages/ \
  && wget https://github.com/ont/slacker/raw/master/handler.py