Adds extra step to Dockerfile build to take advantage of layer caching and avoid repulling dependencies in requirements.txt if they don't change

.ci/Dockerfile

@@ -2,6 +2,9 @@ ARG PYTHON_VERSION=3
 FROM python:${PYTHON_VERSION}
 
 WORKDIR /code/eland
+COPY requirements-dev.txt .
+RUN pip install -r requirements-dev.txt
+
 COPY . .
 RUN chmod +x ./run_build.sh
-RUN pip install -r requirements-dev.txt
+