Do not store pip cache during docker build

[don-attilio]

Hi, thanks for developing the guardrails.
Removing the pip cache in a layer subsequent to the pip install does not reduce image size because the cache stays in the final image. It is more effective and cleaner not using any caching at all.

Adding the option --no-cache-dir during pip command reduces the final artifact size for each of the Dockerfiles in the repo, like listed below.

1. Dockerfile: from 1.68GB to 1.58GB;
2. nemoguardrails/library/jailbreak_detection/Dockerfile: from 12.3GB to 9.37GB;
3. nemoguardrails/library/jailbreak_detection/Dockerfile-GPU: from 12.3GB to 9.41GB;
4. nemoguardrails/library/factchecking/align_score/Dockerfile: from 9.6GB to 7.6GB;

For the last three dockerfiles, I reordered the COPY and RUN directives in order to avoid having to download the dependencies every time a single line of code changes.
There could be definitely be more optimizations, such as multi-layer builds.

[drazvan]

Thanks @don-attilio!
👍