It's as easy as pointing out another folder in the startup script. Change the path between the -v and the first colon to the folder where your dashing project lies.
$ sudo docker run -i -d -p 3030:3030 --name dashing -v /your/path/to/dashing-proj:/dashboard jcastillo/dashing
Copy or rewrite the start.sh script in the sample dashboard folder, it has to be in your dashing folder. It's purpose is to activate the dashing service.
This is the default content of the start.sh file:
cd dashboard bundle dashing start