support pack bindings

[taraszka]

Description of feature

Following https://paketo.io/docs/howto/configuration/#bindings, the current dokku implementation lacks pack volume bindings.

[taraszka]

@josegonzalez, I think volume binding during the build phase may require a completely different approach. What do you think? (This https://dokku.com/docs/advanced-usage/persistent-storage/#build-phase will work partially with pack; another thing is that param is required for the pack CLI to make it aware.

I am happy to code this feature myself, but I need some guidance, as storage is managed under storage:<command> <app>, builder has its own namespace builder:set, and the app it owns.

How about builder-pack:binding <app> <host-dir:container-dir> in this case?

[josegonzalez]

What if we just ignored storage mounts during a pack build unless the container path has the prefix /platform/bindings/ (or whatever the configured SERVICE_BINDING_ROOT env var is), and then pass those along to pack as bindings? That would make it so Dokku doesn't need anything extra and the builder-pack plugin just does the right thing.

[josegonzalez]

Yeah the more I think about this, the more I think the existing storage plugin should work fine here. It is possible to set storage specifically for the build process and we currently do not filter volumes.

The big issue is that the pack binary doesn't support the -v shorthand flag (which we use in Dokku, ugh sorry) on the build subcommand. This means we'd need to translate them first - probably in the docker-options#docker-args-build trigger. Basically split the values and if we see the flag name is -v, then change it to --volume.

[taraszka]

Right, it sounds good to me, too! I'll try to have a look at it during the weekend.