Access data in worker

[Didayolo]

Mount $HOST_DIRECTORY/data to /app/data in the container so it can be accessed by ingestion and scoring programs.

This can be used to store data directly inside the compute workers.

See:

• Data inside workers #874

Review:

• Tested and it work
• CircleCI tests are passing
• Documentation is updated

Next steps:

• Merge PR
• Update docker images (cpu and gpu)

[dtuantran]

It's good, one 👍 for merging

[Didayolo]

@dtuantran we still need to update the docker images

[Didayolo]

We should maybe fix #748 before updating the dockers.