16.3.3 .env variables are undefined in docker build #97959

Link to the code that reproduces this issue

https://github.com/VojtechVidra/next-16.3.3-env-docker-issue

To Reproduce

1. Build the application with docker docker build . -t next-env-issue
2. Run the built image docker run -p 3000:3000 next-env-issue
3. Open the route file in browser http://localhost:3000/api/env

Current vs. Expected behavior

Current:

When I build with docker build and run it, the endpoint is returning null for both env variables.

Expected:

The variables are defined even when using docker build.

Provide environment information

Operating System:
  Platform: darwin
  Arch: arm64
  Version: Darwin Kernel Version 25.6.0: Fri Jul 31 19:18:49 PDT 2026; root:xnu-12377.161.14~5/RELEASE_ARM64_T6000
  Available memory (MB): 32768
  Available CPU cores: 10
Binaries:
  Node: 24.19.0
  npm: 11.17.0
  Yarn: N/A
  pnpm: 11.23.0
Relevant Packages:
  next: 16.3.3 // Latest available version is detected (16.3.3).
  eslint-config-next: N/A
  react: 19.2.3
  react-dom: 19.2.3
  typescript: 5.9.3
Next.js Config:
  output: standalone

Which area(s) are affected? (Select all that apply)

Runtime

Which stage(s) are affected? (Select all that apply)

Other (Deployed)

Additional context

No response