docs: Describe volume setup for local Docker builds

README.md

@@ -32,8 +32,12 @@ To build the code vectors for the ICD-9 codelist for the deployed environment:
 
 ## Run in Docker
 
-1. Build the image with: `docker build . -t headwayio/medical_transcription`
-2. Run a container with: `docker run --env-file ./.env -p 4000:4000 headwayio/medical_transcription`
+1. Create a local volume: `docker volume create ml-data`
+2. Ensure the volume is writeable: `docker run --rm -v ml-data:/data busybox /bin/sh -c 'touch /data/.initialized && chmod 1777 /data'`. More background:
+  - What the initial `1` for `chmod` means: https://www.linuxnix.com/sticky-bit-set-linux/
+  - How to update the permissions of a mounted volume: https://serverfault.com/a/984599
+3. Build the image with: `docker build . -t headwayio/medical_transcription`
+4. Run a container with: `docker run --env-file ./.env -p 4000:4000 headwayio/medical_transcription`
 
 ## Learn more