dockerfile: changed image to slim version; added suppress warnings on pip install

Dockerfile

@@ -1,10 +1,10 @@
-FROM python:3.9.15-buster
+FROM python:3.9.15-slim-buster
 
 RUN apt-get update
 RUN apt-get install -y \
 	android-tools-adb \
 	android-tools-fastboot
-RUN pip3 install protobuf
+RUN pip3 install protobuf -q
 COPY . /magisk
 WORKDIR /magisk
 CMD bash reinstall-magisk-on-lineageos