improve dockerfile;

backend/Dockerfile

@@ -1,19 +1,21 @@
 FROM python:3.8-slim
+
 ENV PYTHONUNBUFFERED 1
-RUN apt-get update
-RUN apt-get install -y build-essential gcc
 
-RUN python -m venv /opt/venv
-ENV PATH="/opt/venv/bin:$PATH"
-
-RUN python -m pip install --upgrade pip
+RUN apt-get update; \
+    apt-get install -y build-essential gcc; \
+    python -m venv /opt/venv; \
+    python -m pip install --upgrade pip;
 
 ENV PATH="/opt/venv/bin:$PATH"
 
 RUN mkdir /code
+
 WORKDIR /code
+
 COPY setup.py /code/
 COPY requirements.txt /code/
+
 RUN pip install -r requirements.txt
 
 COPY . /code/