If, when doing something like docker-compose up, you run into an error like this:

ERROR: Named volume "server/db:/usr/share/nginx/html/db:rw" is used in service "web" but no declaration was found in the volumes section.

You're missing a leading ./, so, for example:

volumes:
 - 'server/db:/usr/share/nginx/html/db'

should read

volumes:
 - './server/db:/usr/share/nginx/html/db'

This example assumes that you want to map a host directory ./server/db/ to a container directory /usr/share/nginx/html/db, not create a named volume server/db.