allow to use environment variable for nextjs rewrites

[jantimon]

Describe the feature you'd like to request

We would like to use a different fallback server per environment e.g.:

module.exports = {
  async rewrites() {
    return {
      fallback: [
        // These rewrites are checked after both pages/public files
        // and dynamic routes are checked
        {
          source: '/:path*',
          destination: process.env.FALLBACK_SERVER,
        },
      ],
    }
  }
}

Right now this is not possible as process.env.FALLBACK_SERVER is processed during build time.
So we are not able to deploy the same image for different environments

Describe the solution you'd like

Allow to evaluate environment variables during runtime e.g. by allowing to define a function for destination:

module.exports = {
  async rewrites() {
    return {
      fallback: [
        // These rewrites are checked after both pages/public files
        // and dynamic routes are checked
        {
          source: '/:path*',
          destination: () => process.env.FALLBACK_SERVER,
        },
      ],
    }
  }
}

Describe alternatives you've considered

the alternative would be to have different builds per environment.
however we would prefer to test the exact same build which will later be deployed to production.

[makeable]

Just got bitten by this one :(

[STS-SW]

How did you solve this?