Generates the fresh-install Docket schema migration.
The generated up/0 installs the current schema version and down/0
removes the Docket schema entirely. Hosts upgrading an existing
installation do not use this task: each release's upgrade guide documents
a hand-written migration pinning Docket.Postgres.Migration.up/1 and
down/1 to the versions that release adds, so a rollback returns the
host to its pre-upgrade schema.