Fix amphtml link rel not respecting basePath (#15949)

Co-authored-by: kodiakhq[bot] <49736102+kodiakhq[bot]@users.noreply.github.com>

packages/next/next-server/server/config.ts

@@ -199,6 +199,10 @@ function assignDefaults(userConfig: { [key: string]: any }) {
         if (result.assetPrefix === '') {
           result.assetPrefix = result.basePath
         }
+
+        if (result.amp.canonicalBase === '') {
+          result.amp.canonicalBase = result.basePath
+        }
       }
     }
   }

test/integration/basepath/pages/amp-hybrid.js

@@ -0,0 +1,3 @@
+export const config = { amp: 'hybrid' }
+
+export default () => <p>Hello amp</p>

test/integration/basepath/test/index.test.js

@@ -1027,6 +1027,13 @@ describe('basePath development', () => {
       })
     })
   })
+
+  it('should respect basePath in amphtml link rel', async () => {
+    const html = await renderViaHTTP(context.appPort, '/docs/amp-hybrid')
+    const $ = cheerio.load(html)
+    const expectedAmpHtmlUrl = '/docs/amp-hybrid?amp=1'
+    expect($('link[rel=amphtml]').first().attr('href')).toBe(expectedAmpHtmlUrl)
+  })
 })
 
 describe('basePath production', () => {
@@ -1053,6 +1060,13 @@ describe('basePath production', () => {
   afterAll(() => stopApp(server))
 
   runTests(context)
+
+  it('should respect basePath in amphtml link rel', async () => {
+    const html = await renderViaHTTP(context.appPort, '/docs/amp-hybrid')
+    const $ = cheerio.load(html)
+    const expectedAmpHtmlUrl = '/docs/amp-hybrid.amp'
+    expect($('link[rel=amphtml]').first().attr('href')).toBe(expectedAmpHtmlUrl)
+  })
 })
 
 describe('basePath serverless', () => {