chore: remove the hardcoded 2.21.3 version in dockerfile (#568)

* remove the hardcoded 2.21.3 version in dockerfile

* Apply suggestions from code review

Co-authored-by: datadog-datadog-prod-us1[bot] <88084959+datadog-datadog-prod-us1[bot]@users.noreply.github.com>

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Co-authored-by: Munir Abdinur <munir.abdinur@datadoghq.com>
Co-authored-by: datadog-datadog-prod-us1[bot] <88084959+datadog-datadog-prod-us1[bot]@users.noreply.github.com>

Author: 3 people

Dockerfile

@@ -14,7 +14,7 @@ ENV PATH=/root/.cargo/bin:$PATH
 
 # Install datadog_lambda and dependencies from local
 COPY . .
-RUN pip install . ddtrace==2.21.3 -t ./python/lib/$runtime/site-packages
+RUN pip install --no-cache-dir . -t ./python/lib/$runtime/site-packages
 
 # Remove botocore (40MB) to reduce package size. aws-xray-sdk
 # installs it, while it's already provided by the Lambda Runtime.