containers: export all images in one file

This change modifies how images are exported before packaging.
Previously, each image was exported as a separate file, which
causes duplication of layers in common between images. Now, all
images are exported as one file, allowing deduplication of image
layers and significant on-disk space savings.

NOTE: the docker-registry image is exported separately for the
sake of engine.

Change-Id: I95a0597590ada9c32c8b9449d6f58b321168c2f8
Signed-off-by: Cian Johnston <cian.johnston@est.tech>