switch docker image to python:slim,

move requirements.txt to .devcontainer/

.devcontainer/Dockerfile

@@ -1,7 +1,6 @@
-FROM ubuntu:20.04
+FROM python:slim
 WORKDIR /
-RUN apt-get -y update
-RUN DEBIAN_FRONTEND=noninteractive apt-get install -y --no-install-recommends tzdata
-RUN apt-get install -y python3 python3-dev git python3-pip python3-kivy python-is-python3 libgl1-mesa-dev build-essential
-RUN ln -s $(which pip3) /usr/bin/pip
-RUN pip install pymongo pandas numpy scipy scikit-learn matplotlib pylint kivy
+RUN apt-get -y update; apt-get -y upgrade
+RUN apt-get -y install git
+COPY requirements.txt .
+RUN pip install -r requirements.txt