Add a user guide on manual examination (without Docker)

[nuest]

We should add a user guide on how to examine an ERC without Docker and without the reproducibility service and platform/UI.
This could mitigate issues about "What if Docker is not available anymore?" and demonstrate that the information is still there and accessible using the structure required by the spec alone.

• unpack the archive
• on the structure of Docker exports (maybe a side note on image squashing)
• where to find the ERC metadata files
• how to extract the ERC payload from /erc
• how to check bag validity on the extracted payload
• how to find the main and view files (default names, erc.yml)
• blog post (examining a Docker image without Docker)

Resources

• http://blog.oddbit.com/2015/02/13/unpacking-docker-images/
• https://github.com/larsks/undocker/ (https://github.com/coaic/undocker3)
• https://docs.docker.com/engine/reference/commandline/image_inspect/
• https://stackoverflow.com/a/22730520