[Installation-Wiki FIXME-1 (diskless mode)] config/package cache on *internal* disk

[ Alpine's "Installation" page covers the Alpinelinux disk modes. (https://wiki.alpinelinux.org/wiki/Installation) However, it still has to refer to two manual setup-alpine "FIXME" workarounds, to cover the basic installation options. ]

=> In setup-alpine, please also show internal partitions in the selection for local configs and package cache in the "diskless" operation mode.

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From the wiki:

[FIXME-1: Storing local configs and the package cache on an internal disk still requires some manual steps to have the partition listed, i.e. making a /etc/fstab entry, mountpoint, and mount before running setup-alpine. And requires to manually commit this configuration to disk afterwards.]

The manual workaround steps are maintained at:

https://wiki.alpinelinux.org/wiki/Alpine_local_backup#Saving_and_loading_ISO_image_customizations