Provide an official docker image for cog

[josegonzalez]

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It would be great to have a docker image for cog so that cog doesn't need to be installed on the host. This would allow me to more easily add support for Cog building into dokku as a builder.

[bfirsh]

Great idea!

There is some prior art here where we tried running Cog in a container for the integration tests, but it expects the prediction server to be on localhost and files to be in particular places on the filesystem, and so on: https://github.com/replicate/cog/pull/384. All fixable things, I think.

We also made this work for Docker Compose by making this rather elaborate wrapper script that sets up all the volumes correctly and so on: https://github.com/docker/compose/blob/py2/script/run/run.sh

[josegonzalez]

Maybe it's worth exposing the host you bind to? Also, what paths does cog write to?