Add note to allow read:packages from GHCR for docker dev setup

[tomalrussell]

docker compose  -f docker-compose.dev.yml up

Complains with unauthorized/denied:

 ! backend Warning          Head "https://ghcr.io/v2/nismod/jsrat-backend/manifests/0.1": unauthorized                                                                   2.8s
 ! vector-tileserver Warning Head "https://ghcr.io/v2/nismod/jsrat-vector-tileserver-dev/manifests/0.1": denied                                                          2.8s
 ! frontend Warning         Head "https://ghcr.io/v2/nismod/jsrat-frontend/manifests/0.1": unauthorized                                                                  2.7s
 ! raster-tile-ingester Warning Head "https://ghcr.io/v2/nismod/jsrat-raster-tileserver/manifests/0.1": unauthorized                                                     2.8s

[tomalrussell]

For pulling from the GitHub container registry (GHCR), you will need to follow these
instructions for authentication
and use a token with read:packages scope to read from the GHCR.