Fix the previews app image build so it deploys again

[joyliu-q]

Deploys of training-gym-previews have been failing since the nginx conf dir moved into the image, so the deployed deploy_preview() predates #491 and dashboard previews still proxy /api at the deployed dashboard instead of the PR's own backend (visible on #489's preview job: Previews app does not accept api_url yet ...; deploying without it).

Modal rejects a build step that runs after a non-copy add_local_*:

An image tried to run a build step after using `image.add_local_*` to include local files.

Two chains did that; both now add the mounted (non-copy) file last:

• module image: add_local_dir(nginx) → add_local_file(astro_redirects, copy=True) becomes add_local_file(..., copy=True) → add_local_dir(nginx)
• docs sandbox image: add_local_file(docs.conf) → add_local_file(astro-refresh.inc, copy=True) becomes the copy=True .inc first, then docs.conf

No behavioral change — same files land at the same paths. Not verifiable locally (hydrating an image needs Modal credentials this box doesn't have); the Previews App deploy on merge is the check.

Checklist

• Example is documented with comments throughout, in a Literate Programming style.
• Example does not require third-party dependencies to be installed locally
• Example pins its dependencies
  • Example pins container images to a stable tag, not a dynamic tag like latest
  • Example specifies a python_version for the base image, if it is used
  • Example pins all dependencies to at least minor version, ~=x.y.z or ==x.y
  • Example dependencies with version < 1 are pinned to patch version, ==0.y.z

Link to Devin session: https://modal.devinenterprise.com/sessions/95e5ad2ac22742e98d76804b1c04343b
Open in Devin Desktop: https://modal.devinenterprise.com/desktop/session/95e5ad2ac22742e98d76804b1c04343b?variant=devin
Requested by: @joyliu-q

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[github-actions]

Dashboard preview: https://modal-labs-training-gym--training-gym-previews-preview-r-78da35.modal.run/495/dashboard