Implement --no-export

[jhunkeler]

Allows one to append data to an existing build without affecting its delivered state.

Possible use case

• Tests must be executed on a powerful server, but the target host does not have all of the required software installed (i.e. docker). The delivered files can be downloaded on a system with docker installed, and stasis --no-export --overwrite can be used to build the image without changing the previously delivered files.