Reorganize Dockerfile to leverage build cache

Add an explicit copy of the requirements file and call the pip install on it
earlier so that on modification of the ocs package we don't have to download
all dependencies again. This'll improve turn around time when iterating on
building OCS Docker images locally for testing during development.

Dockerfile

@@ -22,11 +22,15 @@ RUN apt-get update && apt-get install -y python3 \
 # Install init system
 RUN pip3 install dumb-init
 
+# Copy in and install requirements
+# This will leverage the cache for rebuilds when modifying OCS, avoiding
+# downloading all the requirements again
+COPY requirements.txt /app/ocs/requirements.txt
+WORKDIR /app/ocs/
+RUN pip3 install -r requirements.txt
+
 # Copy the current directory contents into the container at /app
 COPY . /app/ocs/
 
-WORKDIR /app/ocs/
-
 # Install ocs
-RUN pip3 install -r requirements.txt && \
-    pip3 install -e .
+RUN pip3 install -e .