What version of Next.js are you using?

10.0.7, 10.0.8-canary.3

What version of Node.js are you using?

15.8.0

What browser are you using?

curl, Chrome

What operating system are you using?

macOS

How are you deploying your application?

next start

Describe the Bug

Custom Cache-Control headers configured in next.config.js are overwritten in some cases. It looks any page that use getStaticProps will have their cache headers overwritten with Cache-Control: s-maxage=31536000, stale-while-revalidate which seems to come from

Expected Behavior

When we configure a Cache-Control header, don't set it to something else.

To Reproduce

Starting from yarn create next-app --example headers headers-app, here's a sample project that demonstrates the bug:
https://github.com/hartshorne/headers-app

You can clone the project, and run something like yarn build && yarn start to start it in production mode. (dev mode overwrites all Cache-Control headers to prevent the browser from caching during development, which makes sense.)

Here's the next.config.js: https://github.com/hartshorne/headers-app/blob/main/next.config.js

Here's props.js (which exports getStaticProps) – this is broken: https://github.com/hartshorne/headers-app/blob/main/pages/props.js

% curl -I http://localhost:3000/props 
HTTP/1.1 200 OK
Cache-Control: s-maxage=31536000, stale-while-revalidate
X-Custom-Header: with static props
X-Powered-By: Next.js
ETag: "978-AzRqkosC2YfRQvbfuY7KiKb51e8"
Content-Type: text/html; charset=utf-8
Content-Length: 2424
Vary: Accept-Encoding
Date: Thu, 18 Feb 2021 22:09:41 GMT
Connection: keep-alive
Keep-Alive: timeout=5

Here's static.js — this works: https://github.com/hartshorne/headers-app/blob/main/pages/props.js

% curl -I http://localhost:3000/static     
HTTP/1.1 200 OK
Cache-Control: public, max-age=3600, s-maxage=60, stale-while-revalidate=86400
X-Custom-Header: no props, no link
X-Powered-By: Next.js
ETag: "945-MX0a4VCO/O001kyDcNn9a55taLQ"
Content-Type: text/html; charset=utf-8
Content-Length: 2373
Vary: Accept-Encoding
Date: Thu, 18 Feb 2021 22:09:10 GMT
Connection: keep-alive
Keep-Alive: timeout=5

Note that X-Custom-Header comes through, but the Cache-Control header is overwritten. Same behavior in Chrome, and with a regular GET request.