[Feature Request] Support "attachments"

[doanac]

I apologize if this is an issue that should raised on the cnab-spec. I'd be happy to open it there if needed.

One of the things that makes docker-compose so amazing is its ability to map current-working directory relative files into a container. The most common use is probably with nginx where you want to mount a single .conf file under /etc/nginx/conf.d to alter the behavior of the service.

I think the usual answer is "build your own container with this file that extends nginx". That's a pain because:

1. Now I have to create a new container
2. I probably don't have to resources to make it multi-arch
3. My container is going to bit-rot and not pick up all the security patches going into nginx

Docker Apps seems to take advantage of stuff layers of content in the the Registry backend, so it seems like its might be possible to also stuff "attachments" into the Registry and link them into the Docker App somehow?

NOTE: You can kind of hack things with settings, but it requires crazy shell hacking to echo the content of the resulting environment variable into a file and then execute the container's "entrypoint".