refactor: Make the Dockerfile a tiny bit cache friendlier

Still not great as the whole installation happens after copying
but at least not copying is not the first thing we do.

Related #124

Dockerfile

@@ -1,22 +1,17 @@
 FROM phusion/baseimage:jammy-1.0.3
 
-WORKDIR /opt/app
-
-COPY . /opt/app
-
-WORKDIR /opt/app/fact_extractor
-
+# Install python and create a virtualenv
 ARG USER=root
 ARG DEBIAN_FRONTEND=noninteractive
-
 RUN install_clean python3.11 python3.11-dev python3.11-venv gcc
+RUN python3.11 -m venv /venv
+ENV PATH=/venv/bin:$PATH
 
-RUN python3.11 -m venv venv
-
-RUN . venv/bin/activate && install/pre_install.sh
-
-RUN . venv/bin/activate && venv/bin/python3.11 install.py
+# Copy and install fact_extractor
+COPY . /opt/app
+WORKDIR /opt/app/fact_extractor
+RUN . /venv/bin/activate && install/pre_install.sh
+RUN . /venv/bin/activate && /venv/bin/python3.11 install.py
 
-ENV PATH=/opt/app/fact_extractor/venv/bin:$PATH
 
 ENTRYPOINT ["./docker_extraction.py"]