Modifies Dockerfile to use multistage builds (#79)

Dockerfile

@@ -1,29 +1,43 @@
-FROM python:3.8-slim
+# pip dependencies install stage
+FROM python:3.8-slim as builder
 
 # rustc compiler would be needed on ARM type devices but theres an issue with some deps not building..
 ARG CRYPTOGRAPHY_DONT_BUILD_RUST=1
 
-COPY requirements.txt /tmp/requirements.txt
+RUN apt-get update && apt-get install -y --no-install-recommends \
+    libssl-dev \
+    libffi-dev \
+    gcc \
+    libc-dev \
+    libxslt-dev \
+    zlib1g-dev \
+    rustc \
+    g++
 
-RUN apt-get update && apt-get install -y libssl-dev libffi-dev gcc libc-dev libxslt-dev zlib1g-dev rustc g++ --no-install-recommends && rm -rf /var/lib/apt/lists/* /var/cache/apt/*
-# Update pip, install requirements, remove rust and dev packages that are no longer needed.
-RUN pip3 install --upgrade pip && pip3 install --no-cache-dir -r /tmp/requirements.txt && apt-get remove rustc *-dev --purge -y
+RUN mkdir /install
+WORKDIR /install
+
+COPY requirements.txt /requirements.txt
+
+RUN pip install --target=/dependencies -r /requirements.txt
+
+# Final image stage
+FROM python:3.8-slim
+
+# https://stackoverflow.com/questions/58701233/docker-logs-erroneously-appears-empty-until-container-stops
+ENV PYTHONUNBUFFERED=1
 
-RUN [ ! -d "/app" ] && mkdir /app
 RUN [ ! -d "/datastore" ] && mkdir /datastore
 
+# Copy modules over to the final image and add their dir to PYTHONPATH
+COPY --from=builder /dependencies /usr/local
+ENV PYTHONPATH=/usr/local
+
 # The actual flask app
 COPY backend /app/backend
-
 # The eventlet server wrapper
 COPY changedetection.py /app/changedetection.py
 
 WORKDIR /app
 
-# https://stackoverflow.com/questions/58701233/docker-logs-erroneously-appears-empty-until-container-stops
-ENV PYTHONUNBUFFERED=1
-
 CMD [ "python", "./changedetection.py" , "-d", "/datastore"]
-
-
-