Change Dockerfiles and docker-compose file

The reason for this change is to reduce the image size for the different containers

backend/Dockerfile

@@ -1,11 +1,19 @@
-FROM python
+FROM python:slim as base
 
+FROM base as builder
+
+RUN mkdir /install
+WORKDIR /install
+
+COPY requirements.txt /requirements.txt
+
+RUN pip install --prefix=/install -r /requirements.txt
+
+FROM base
+
+COPY --from=builder /install /usr/local
+
+COPY /src /app
 WORKDIR /app
 
-COPY requirements.txt .
-
-RUN pip install -r requirements.txt
-
-COPY /src .
-
 CMD [ "flask", "run" ]