(DEPLOY): speed up HF rebuilds by caching deps

- Install 'requirements.txt' before copying source so code changes don't re-install deps

Dockerfile

@@ -11,9 +11,8 @@ RUN apt-get update && apt-get install -y \
 	&& rm -rf /var/lib/apt/lists/*
 
 COPY requirements.txt ./
-COPY . .
-
 RUN pip3 install -r requirements.txt
+COPY . .
 
 EXPOSE 8501