Determine best way to handle keys in docker vs dev

[mattcollier]

Docker users are able to access key material that is on their local system (not inside the docker container). This is accomplished by mapping a Docker volume.

The current docker image is based on the build-docker branch which has this one commit: 1ec6c2c

The instructions in vc-demo instruct docker users to map a volume from pwd to the key-file folder in the image. https://github.com/digitalbazaar/vc-demo#create-an-alias

Ideally, keys could be handled in a similar way between docker and dev.