It works with /app/src.
This command worked for me:
docker run -p 8090:8080 -e PLONE_DEVELOP="src/my.content" -e ADDONS="my.content" -e SITE="Plone" -e TYPE="classic" -v $(pwd)/src:/app/src plone/plone-backend:6.0.0a4
The only thing non-standard is that I use port 8090 instead of the more common 8080.