Implements auto-linking container images from the package registry to a repository (closes#2823). This might ease implementing #2699 in the future.
Specifically, auto-linking happens on package creation and NOT when publishing updates to the same package. This should prevent "relinking" a manually unlinked package when publishing an update. Linking is performed either via the the Docker label `` (as described here: https://codeberg.org/forgejo/forgejo/issues/2823#issuecomment-8163866) or by naming the image like the repository (supports nested image names).
## Checklist
The [contributor guide](https://forgejo.org/docs/next/contributor/) contains information that will be helpful to first time contributors. There also are a few [conditions for merging Pull Requests in Forgejo repositories](https://codeberg.org/forgejo/governance/src/branch/main/PullRequestsAgreement.md). You are also welcome to join the [Forgejo development chatroom](https://matrix.to/#/#forgejo-development:matrix.org).
### Tests
- I added test coverage for Go changes...
- ~~[ ] in their respective `*_test.go` for unit tests.~~ _(Not required, since only already tested functions were used)_
- [x] in the `tests/integration` directory if it involves interactions with a live Forgejo server.
- ~~I added test coverage for JavaScript changes...~~ _(No changes to JavaScript code)_
### Documentation
- [X] I created a pull request [to the documentation](https://codeberg.org/forgejo/docs) to explain to Forgejo users how to use this change: https://codeberg.org/forgejo/docs/pulls/1666
- [ ] I did not document these changes and I do not expect someone else to do it.
### Release notes
- [ ] I do not want this change to show in the release notes.
- [X] I want the title to show in the release notes with a link to this pull request.
- [ ] I want the content of the `release-notes/<pull request number>.md` to be be used for the release notes instead of the title.
Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/10617
Reviewed-by: Mathieu Fenniak <mfenniak@noreply.codeberg.org>
Co-authored-by: Leon Schmidt <mail@leon.wtf>
Co-committed-by: Leon Schmidt <mail@leon.wtf>
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