How to mount HDFS in a Docker container
You might have a look at the Docker volume docs.
Basically, the volumes definition in the app.json would trigger the start of the Docker image with the flag -v /hadoop/hdfs-mount:/home:RW, meaning that the host path gets mapped to the Docker container as /home in read-write mode.
You should be able to verify this if you SSH into the node which is running the app and do a docker inspect <containerId>.
See also