Update dockerfile to multistage

This change reduce the image size from 727Mb to 469Mb.

Dockerfile

@@ -1,24 +1,40 @@
-FROM python:3.8.6-slim-buster
+FROM python:3.8.6-slim-buster as base
 
-RUN apt-get update \
-    && apt-get -y install curl build-essential libssl-dev sqlite3 \
-    && apt-get clean \
-    && pip install --upgrade pip
+# Setup env
+ENV LANG C.UTF-8
+ENV LC_ALL C.UTF-8
+ENV PYTHONDONTWRITEBYTECODE 1
+ENV PYTHONFAULTHANDLER 1
 
 # Prepare environment
 RUN mkdir /freqtrade
 WORKDIR /freqtrade
 
+# Install dependencies
+FROM base as python-deps
+RUN apt-get update \
+    && apt-get -y install git curl build-essential libssl-dev \
+    && apt-get clean \
+    && pip install --upgrade pip
+
 # Install TA-lib
 COPY build_helpers/* /tmp/
 RUN cd /tmp && /tmp/install_ta-lib.sh && rm -r /tmp/*ta-lib*
-
 ENV LD_LIBRARY_PATH /usr/local/lib
 
 # Install dependencies
 COPY requirements.txt requirements-hyperopt.txt /freqtrade/
-RUN pip install numpy --no-cache-dir \
-  && pip install -r requirements-hyperopt.txt --no-cache-dir
+RUN  pip install --user --no-cache-dir numpy \
+  && pip install --user --no-cache-dir -r requirements-hyperopt.txt
+
+# Copy dependencies to runtime-image
+FROM base as runtime-image
+COPY --from=python-deps /usr/local/lib /usr/local/lib
+ENV LD_LIBRARY_PATH /usr/local/lib
+
+COPY --from=python-deps /root/.local /root/.local
+ENV PATH=/root/.local/bin:$PATH
+
 
 # Install and execute
 COPY . /freqtrade/