Updated Docker file for fastest rebuild

Dockerfile

@@ -2,10 +2,12 @@ FROM python:3.6
 ENV PYTHONUNBUFFERED 1
 RUN mkdir /lilcity
 WORKDIR /lilcity
-ADD requirements.txt /lilcity/
 RUN apt-get update \
     && apt-get install -y postgresql-client-9.4 \
     && rm -rf /var/lib/apt/lists/* \
-    && pip install --upgrade pip \
+    && pip install --upgrade pip
-    && pip install -r requirements.txt
+
+ADD requirements.txt /lilcity/
+RUN pip install -r requirements.txt
+
 ADD . /lilcity/