docker: use slim and add multiple layers

reduces the image's size with over 80%

docker/Dockerfile

@@ -1,13 +1,20 @@
-FROM python:3.12-bookworm
 LABEL maintainer="darius.stefan@opensips.org"
 
-RUN apt update && apt install -y tcpdump &&\
-    pip install --upgrade pip &&\
-    pip install pyyaml &&\
-    git clone https://github.com/OpenSIPS/SIPssert.git &&\
+FROM python:3.12-slim-bookworm AS build
+
+RUN apt update && apt install -y git
+
+WORKDIR /usr/src
+RUN git clone https://github.com/OpenSIPS/SIPssert.git &&\
     cd SIPssert &&\
-    python3 setup.py install &&\
-    cd ..
+    python3 setup.py install
+
+FROM python:3.12-slim-bookworm AS sipssert
+
+RUN apt update && apt install -y tcpdump
+
+COPY --from=build /usr/local/bin/sipssert /usr/local/bin/sipssert
+COPY --from=build /usr/local/lib/python3.12/site-packages /usr/local/lib/python3.12/site-packages
 
 ENTRYPOINT ["sipssert"]