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>

elastalert/Dockerfile

@@ -25,7 +25,7 @@ RUN set -xe \
         musl-dev \
         openssl-dev \
         python3-dev \
-    && pip install elastalert==${ELASTALERT_VERSION} \
+    && pip install --no-cache-dir elastalert==${ELASTALERT_VERSION} \
     && mkdir -p rules \
     && curl -sSL https://github.com/Yelp/elastalert/raw/${ELASTALERT_VERSION}/config.yaml.example > config.yaml \
     && apk del .build-deps