CI: workflow to build container image nightly

[Zerpet]

Proposed Changes

It's a middle ground between building on every commit, and not building
at all. We currently have a workflow to build the OCI on every PR.
However, building on every PR does not cover some use cases. For
example, providing an image to a user to preview some changes coming in
the next minor or patch. Another use case: compare main with a PR in
Kubernetes.

It's better to have a separate workflow, even at the expense of
duplication, because the "on schedule" trigger only runs on the default
branch of the repository. This "limitation" makes it complicated to
extend the current "build OCI on PRs" to also build nightly for main and
release branches.

Types of Changes

What types of changes does your code introduce to this project?
Put an x in the boxes that apply

• Bug fix (non-breaking change which fixes issue #NNNN)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause an observable behavior change in existing systems)
• Documentation improvements (corrections, new content, etc)
• Cosmetic change (whitespace, formatting, etc)
• Build system and/or CI

[Zerpet]

This doesn't need backporting because "on schedule" only runs on the default branch i.e. main