Update Dockerfile

backend/Dockerfile

@@ -1,5 +1,5 @@
 # syntax=docker/dockerfile:1
-FROM python:3.9-alpine AS build
+FROM python:3.7-alpine AS build
 ARG EXTRA_DEPS
 RUN apk add build-base musl-dev libffi-dev openssl-dev mariadb-dev
 WORKDIR /app
@@ -8,7 +8,7 @@ COPY pyproject.toml poetry.lock ./
 RUN poetry config virtualenvs.create false
 RUN poetry install --no-dev --no-ansi --no-interaction
 
-FROM node:12-alpine AS package
+FROM python:3.7-alpine AS package
 WORKDIR /app
 COPY --from=0 /usr /usr
 COPY manage.py gunicorn.conf.py ./