bug fix - On queries with no labels, adding the cluster label with a
comma prefix created an invalid query ( `machine_memory_bytes{,
cluster="foo"}` ) that failed

Fixes #325

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using the "--no-cache-dir" flag in pip install, make sure downloaded
packages by pip don't cache on the system. This is a best practice that
makes sure to fetch from a repo instead of using a local cached one.
Further, in the case of Docker Containers, by restricting caching, we
can reduce image size. In terms 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 reduces a lot by don't cache pip
packages.

Further, more detailed information can be found at


https://medium.com/sciforce/strategies-of-docker-images-optimization-2ca9cc5719b6

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Signed-off-by: Pratik Raj <rajpratik71@gmail.com>

### Tests ran

- Ran krr before the upgrade and after the upgrade to compare the output

…343)