Update Dockerfile with requirements caching

Dockerfile

@@ -1,8 +1,10 @@
 FROM ubuntu:latest
 
+WORKDIR /subgen
+
 RUN apt-get update && apt-get -y install python3 python3-pip
 
-ADD https://raw.githubusercontent.com/McCloudS/subgen/main/subgen/subgen.py /subgen/subgen.py
+RUN pip install -r requirements.txt
 
 ENTRYPOINT ["python3"]
 CMD ["/subgen/subgen.py"]