change docker base image to python slim

Author: vgvoleg

tests/slo/Dockerfile

@@ -1,7 +1,9 @@
-FROM python:3.8
+FROM python:3.8-slim
 COPY . /src
 WORKDIR /src
-RUN python -m pip install --upgrade pip && python -m pip install -e . && python -m pip install -r tests/slo/requirements.txt
+RUN python -m pip install --no-cache-dir --upgrade pip && \
+    python -m pip install --no-cache-dir -e . && \
+    python -m pip install --no-cache-dir -r tests/slo/requirements.txt
 WORKDIR tests/slo
 ARG SDK_SERVICE
 ENV SDK_SERVICE=$SDK_SERVICE