fix(core): docusaurus serve redirects should include the site /baseUrl/ prefix

[slorber]

Motivation

Fix #10078

docusaurus serve depends on vercel/serve-handler

This package is not super maintained and quite annoying to work with, particularly when we need to serve a static deployment under a baseUrl.

We can't do a rewrite from /baseUrl/:path to /:path, all requests end up as 404. I think the destination path must be an existing HTML files, not just a pathname.

Historically we have worked around that limitation thanks to the following hack:

    // Remove baseUrl before calling serveHandler, because /baseUrl/ should
    // serve /build/index.html, not /build/baseUrl/index.html (does not exist)
    req.url = req.url.replace(baseUrl, '/');

But the problem is that when the server handler is called with a trailingSlash: false setting for example, the responses will redirect /baseUrl/xyz/ to /xyz instead of /baseUrl/xyz. And the lib calls res.end() so there's no way to properly re-integrate that baseUrl to the redirect Location header.

So, this fix applies another workaround on top of serve-handler, so that we can avoid incorrect "trailing slash redirects" from serve-handler.

Test Plan

It's hard to test this automatically 😅 but I tested this locally under various scenarios

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⚡️ Lighthouse report for the deploy preview of this PR

URL	Performance	Accessibility	Best Practices	SEO	PWA	Report
/	🟠 72	🟢 98	🟢 96	🟢 100	🟠 88	Report
/docs/installation	🟠 58	🟢 96	🟢 100	🟢 100	🟠 88	Report
/docs/category/getting-started	🟠 75	🟢 100	🟢 100	🟢 90	🟠 88	Report
/blog	🟠 70	🟢 100	🟢 100	🟢 90	🟠 88	Report
/blog/preparing-your-site-for-docusaurus-v3	🟠 61	🟢 96	🟢 100	🟢 100	🟠 88	Report
/blog/tags/release	🟠 71	🟢 100	🟢 100	🟠 80	🟠 88	Report
/blog/tags	🟠 75	🟢 100	🟢 100	🟢 90	🟠 88	Report

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Size Change: 0 B

Total Size: 1.59 MB

ℹ️ View Unchanged

Filename	Size
website/.docusaurus/codeTranslations.json	2 B
website/.docusaurus/docusaurus.config.mjs	26.7 kB
website/.docusaurus/globalData.json	91.2 kB
website/.docusaurus/i18n.json	930 B
website/.docusaurus/registry.js	247 kB
website/.docusaurus/routes.js	156 kB
website/.docusaurus/routesChunkNames.json	109 kB
website/.docusaurus/site-metadata.json	2.17 kB
website/build/assets/css/styles.********.css	113 kB
website/build/assets/js/main.********.js	810 kB
website/build/index.html	38 kB

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[msyfls123]

@slorber

We can't do a rewrite from /baseUrl/:path to /:path, all requests end up as 404. I think the destination path must be an existing HTML files, not just a pathname.

I'm afraid that serve command failed to serve static files like 'index.js' with trailingSlash: true since it will always redirect to a 404 page.

[slorber]

@msyfls123 if you think there's a bug please create a docusaurus.new repro with clear instructions on how to see the wrong behavior happening, and what is the expected behavior

[slorber]

@msyfls123 probably this bug: #10139