Reorder Dockerfile to improve build times

Although the original Dockerfile was quite simple and straightforward,
it wasn't necessarily fast for doing repeated builds (local testing).

By putting things first which rarely change (pip install of base
requirements, for example), we can take advantage of docker caching to
skip redundant build steps.

Change-Id: I2fd961d8ad5c35280ccb23b00767acb6b5269993

Dockerfile

@@ -52,8 +52,7 @@ RUN pip3 install virtualenv
 # Expose port
 EXPOSE 8080 3306
 
-# Add Craton
-ADD . /craton
+Add ./requirements.txt /requirements.txt
 
 # Init virutalenv
 RUN virtualenv -p /usr/bin/python3.5 /craton
@@ -61,11 +60,14 @@ RUN virtualenv -p /usr/bin/python3.5 /craton
 # Change Working Dir
 WORKDIR /craton
 
+# Install requirements
+RUN bin/pip install -r /requirements.txt
+
 # pip install mysql-python
 RUN bin/pip install mysqlclient
 
-# Install requirements
-RUN bin/pip install -r /craton/requirements.txt
+# Add Craton
+ADD . /craton
 
 # Install Craton
 RUN bin/pip install .