Add weekly scheduled rebuild for UV-based images (closes #203)

[ericof]

Summary

Wire up the weekly rebuild promised in #203:

• Add .github/workflows/schedule-uv.yml on the default branch. It fires weekly on Mondays at 05:00 UTC (and can be triggered manually) and dispatches release-uv.yml on the uv branch via gh workflow run ... --ref uv.
• Document the UV-based images and their weekly refresh cadence in README.md.

Why a dispatcher workflow instead of a schedule: on release-uv.yml directly?

GitHub Actions only fires schedule: triggers from workflow files on the default branch. release-uv.yml lives on the uv branch, so a schedule: trigger added there would silently never fire. Landing the schedule on 6.1.x and having it dispatch the uv-branch workflow via workflow_dispatch keeps the build logic where the source lives and avoids duplicating the matrix build on two branches.

The dispatch step relies on a documented exception: workflow_dispatch events fired by the default GITHUB_TOKEN do start new workflow runs (most other token-triggered events don't). No PAT is required — just permissions: actions: write.

Tag strategy

image-release.yml already tags images as plone/server-{builder,prod-config}:uv-<python>, so scheduled rebuilds transparently overwrite the existing tags. Consumers pulling uv-3.13 always get the most recent rebuild, which matches the goal of "stay in sync with upstream".

Test plan

• Merge this PR and confirm Schedule UV image rebuild appears in the Actions tab on 6.1.x.
• Manually run the workflow via workflow_dispatch and confirm it successfully dispatches release-uv.yml on the uv ref.
• After the dispatched build completes, verify fresh digests for plone/server-builder:uv-3.12 (and siblings) on GHCR and Docker Hub.
• Observe the first real scheduled run on the following Monday and confirm images rebuild without intervention.

Closes #203

[mauritsvanrees]

I guess once 6.2.0 final has been released, we should switch this action workflow to the 6.2.x branch. For now, I have only copied the README change and added it in my PR #210 for 6.2.0rc2.

[davisagli]

@ericof @mauritsvanrees I think we should also keep updating the 6.1 image weekly as long as 6.1 is considered to be in security support.

[ericof]

@ericof @mauritsvanrees I think we should also keep updating the 6.1 image weekly as long as 6.1 is considered to be in security support.

I think we should do it, but this is going to be a major refactoring (if we want to do it for tags)
If we do it only for 6.1, we could do it from the branch.