refactor: Make project column nullable and improve container registry migration script

[jopemachine]

Follow-up of #1917.

• Make image table's project column nullable
  • If the "project" column is null, the string after the registry will be treated as the image name, following the existing logic.
• Make container registry table's project column nullable
  • This is primarily intended for more flexible support for local-type container registries, but it may also be useful for other use cases. (See fix: Typo when query container registry #2890.)
• Modified the upgrade() to automatically generate the corresponding container registry row instead of raising an error when no container registry exists for the image canonical name.

Thoughts about local type container registry record

• Local-type container registry records currently do not contain meaningful information. Additionally, having more than one local container registry type would be awkward.

• Whether an image is local can be determined by checking if the container registry pointed to by image.registry_id is of the Local type, making the is_local column redundant.

Container registry row auto-generation example

🕙 18:58:36 ❯ ./py -m alembic upgrade +1
INFO  [alembic.runtime.migration] Context impl PostgresqlImpl.
INFO  [alembic.runtime.migration] Will assume transactional DDL.
INFO  [alembic.runtime.migration] Running upgrade 20218a73401b -> 1d42c726d8a3, Migrate container registry config storage from `Etcd` to `PostgreSQL`
INFO  [alembic.runtime.migration] Following container registry row auto-generated: "cr.backend.ai" with the project "multiarch".
INFO  [alembic.runtime.migration] If credential required for the auto-generated container registry rows, you should fill their credential columns manually.

Caution

This PR contains updates to a revision script which has already been published to the stable release. Existing clusters operating Backend.AI 24.09 should execute following script to manually elaborate the database schema:

alembic downgrade 20218a73401b
alembic upgrade head

Checklist: (if applicable)

• Milestone metadata specifying the target backport version
• Installer updates including:
  • Fixtures for db schema changes
  • New mandatory config options

[jopemachine]

• feat(BA-470): Improve local type container registry handling #2980
• feat(BA-72): Enable image rescanning by project #3237
• refactor(BA-462): Container registry project traversal logic of image rescanning #2979
• refactor: Make project column nullable and improve container registry migration script #2978 👈 (View in Graphite)
• main

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