Editing Tutorials:container which trains MNIST using Tensorflow (section)

== Remark: persistent storage in the container ==

A container brings its own temporary file system, everything which is written to it will by default not impact the host. After the container terminates, all data which was stored on this temporary filesystem is lost. To write to persistent storage from inside the container which survives container destruction, you have to mount a host filesystem. This can be done in the docker-compose.yml like this (add to the "application" container section):

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Everything which is written inside the container to "application/output" will now end up the in the "output" directory on the host (paths are relative to the docker compose config). Vice versa, whatever was/is put into the host directory is now available inside the container. You can also mount read-only if you just want to import data from the host:

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Note that this mechanism is only explained for reference and your own testing, it will not work on the cluster. See the detailed tutorial on Persistent data on the cluster for how it works here.