Explicitly copy requirements into container

docker/Dockerfile

@@ -9,6 +9,7 @@ FROM python:3.8 AS builder
 ARG DEBIAN_FRONTEND=noninteractive
 WORKDIR /opt/HistoQC
 COPY . .
+COPY requirements.txt .
 # Create virtual environment for this project. This makes it easier to copy the Python
 # installation into the second stage of the build.
 ENV PATH="/opt/HistoQC/venv/bin:$PATH"