Turn dockerfile into a multi-stage build

Using a multi-stage build reduces the image size from 1.23GB to 331MB
which means that any node running doccano can now start-up faster.
Additionally, the multi-stage build means that we no longer include
development tools like node or gcc in the production image which reduces
the attack surface of the image.

Dockerfile

@@ -1,5 +1,5 @@
 ARG PYTHON_VERSION="3.6"
-FROM python:${PYTHON_VERSION}
+FROM python:${PYTHON_VERSION} AS builder
 
 ARG NODE_VERSION="8.x"
 RUN curl -sL "https://deb.nodesource.com/setup_${NODE_VERSION}" | bash - \
@@ -7,18 +7,27 @@ RUN curl -sL "https://deb.nodesource.com/setup_${NODE_VERSION}" | bash - \
  && rm -rf /var/lib/apt/lists/*
 
 COPY app/server/package*.json /doccano/app/server/
-RUN cd /doccano/app/server && npm ci
+RUN cd /doccano/app/server \
+ && npm ci
 
 COPY requirements.txt /
-RUN pip install --no-cache-dir -r /requirements.txt
+RUN pip install -r /requirements.txt \
+ && pip wheel -r /requirements.txt -w /deps
 
 COPY app/server/static /doccano/app/server/static/
 COPY app/server/webpack.config.js /doccano/app/server/
-RUN cd /doccano/app/server && DEBUG=False npm run build
+RUN cd /doccano/app/server \
+ && DEBUG=False npm run build \
+ && rm -rf node_modules/
 
 COPY . /doccano
 
-WORKDIR /doccano
+FROM python:${PYTHON_VERSION}-slim AS runtime
+
+COPY --from=builder /deps /deps
+RUN pip install --no-cache-dir /deps/*.whl
+
+COPY --from=builder /doccano /doccano
 
 ENV DEBUG="True"
 ENV SECRET_KEY="change-me-in-production"
@@ -27,6 +36,7 @@ ENV WORKERS="2"
 ENV GOOGLE_TRACKING_ID=""
 ENV AZURE_APPINSIGHTS_IKEY=""
 
+WORKDIR /doccano
 EXPOSE ${PORT}
 
 CMD ["/doccano/tools/run.sh"]