Runtime custom headers

[AnatoleLucet]

Describe the feature you'd like to request

I'd be nice to have a way to set custom headers at runtime via the Next config file.

Describe the solution you'd like

I think this should be en enhancement of the already existing headers function. I think this function should be reused/reinstantiated at runtime.

For example, we're adding custom headers using some values from environment variables. Because the headers function is only called when building the site (or at least I guess it is), we can't change the headers' value without rebuilding the site.

Describe alternatives you've considered

Currently, we're adding headers with an Express custom server.

[marcklingen]

Currently struggling with this as well. Context: we publish our application as a docker images and users should be able to configure CSP headers via environment variables. Currently they need to build the container from source in order to achieve this, it'd be nice to be able to configure this at runtime.