Use repository URL in README Docker build examples for remote builds

[MaxWindt]

Summary

• Update README.md Docker build instructions to use the repository git URL:
    from: docker build -t copilot-api .
    to: docker build -t copilot-api https://github.com/ericc-ch/copilot-api.git#master
  

• Apply the same change for the GH_TOKEN build-arg example.
  

Why

• Building with a git URL in docker build allows users and CI systems to build the image directly from the remote repository without needing a local clone as the build context.
  

• Makes the examples more explicit and helpful for remote/automated build environments.