docs: update documentation index

README.md

@@ -5,4 +5,7 @@ The app: **https://open-targets-metrics.streamlit.app/**
 This repository contains modules for calculating and visualising Open Targets release metrics, as well as some helper scripts. See instructions for:
 * [Metric calculation](docs/metric-calculation.md)
 * [Metric visualisation and comparison using a Streamlit app](docs/metric-visualisation.md)
+
+Development instructions or specific use cases:
+* [Building a Docker image](docs/building-docker-image.md)
 * [Evidence string comparison](docs/evidence-string-comparison.md)