The docker-options DOCKER_OPTIONS_<PHASE>.migrated sentinels and the storage data/storage-registry/migrations/<app> flag files are now stored as per-app properties (`migrated-<phase>` and `legacy-mounts-migrated` respectively), so they survive a property-store backup/restore and align with the existing `migrated-from-files`, `env-migrated`, and `nginx-conf-sigil-migrated` patterns. The new markers are written only when a non-empty legacy source was actually migrated, distinguishing apps that never had legacy state from apps that did and were drained. A one-cycle upgrade-cycle conversion drains any leftover filesystem sentinels into the new properties and removes the files; for docker-options the conversion deliberately runs before the global `migrated-from-files` short-circuit so installs upgrading from the previous release still get their `.migrated` files converted.
Multi-flag inputs (e.g. `--build-arg X=Y --link a --link b`) used to be stored as a single line, which bypassed the per-line filter that drops `--link` and similar flags for dockerfile-based builders. Each `--flag [value]` group is now stored as its own entry, and a `--process` typed after the app name is lifted into the subcommand flag instead of being stored as a docker option.
Adds a `--process` flag (repeatable) to docker-options:add/remove/clear/list and the new docker-options:list subcommand for querying a single process+phase pair. Process scoping is supported only for the deploy phase since build runs once per app and run covers ad-hoc commands and cron tasks where no Procfile process type is available. Storage moves from `$DOKKU_ROOT/$APP/DOCKER_OPTIONS_*` files to property lists under `/var/lib/dokku/config/docker-options/$APP/{processType}.{phase}`, with `_default_` as the sentinel for app-wide options. The install trigger migrates pre-existing DOCKER_OPTIONS_* files into property lists once and renames them to `.migrated`; a global marker makes re-runs strictly no-op. The legacy docker-args-{build,deploy,run} bash triggers are reimplemented in Go alongside a new docker-args-process-deploy trigger that surfaces per-process options to the scheduler. The :report command exposes one dynamic flag per configured `process.deploy` pair (e.g. `--docker-options-deploy.web`) and supports `--format json`. There is no `--global` flag; omitting `--process` keeps the historical default behaviour, since `--global` elsewhere in dokku means "across all apps". Closes#2441.
