Provide debug docker images to the able to run other commands inside the container

Line 38 in 9aa8332

docker exec -it shiori sh

There's no sh in the image currently, so this command gives an error. Maybe change it to

docker exec -it shiori [command], where [command] is something like shiori pocket 1.html.

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Relates to #964

How can the export or import function be used, as we are currently unable to exec into the container?
I remember in past versions it was possible to docker exec -it shiori /bin/ash into the container as it was alpine based.
Is there a possibility that we can currently use the export function?