docs: Troubleshooting proxy build errors

[christopher-hakkaart]

• Explain how to interpret and troubleshoot Wave build errors when builds fail due to errors container registries behind reverse proxies, specifically Cloudflare EDU-1108

[gavinelder]

I am unsure about the appropriateness of this documentation.

From the EDU Ticket

The Wave documentation does not explain how to interpret or troubleshoot Wave build errors, particularly when builds fail due to errors propagating from customer container registries behind reverse proxies like Cloudflare. The WAVE build page currently displays only a single-line error message, which obscures the root cause (e.g., whether the error originates from the customer's ACR, Cloudflare, or buildkit).

In this specific instance the customers infrastructure was returning an error which was conveyed in the build logs.

This PR documents the troubleshooting steps taken to confirm the customers infrastructure provider due to not being aware of the internal repository configuration which is outside of our support boundary.

I would personally limit the documentation to if you see the following in the build logs

#9 1267.4 error: failed to copy: unexpected status from PUT request to https:/xxxx: 520 <none>

This indicates an error sending file contents to the upstream repository, please contact the upstream repository owner for support and guidance.

[christopher-hakkaart]

I would personally limit the documentation to if you see the following in the build logs

All good, I'll strip this back to something that describes the error and to contact the repository owner for support and guidance