Files are copied instead of symlinked.

[gety9]

I am using using "magento-deploystrategy": "symlink", but files are copied.
What can be the reason?

Here are the settings i use:

main composer.json

{
  "name": "myproject/trt",
  "require": {
    "myname/magento-core-composer-installer": "*",
    "openmage/magento-lts": "1.9.4.x-dev",
    "myproject/trt-source": "@dev"
  },
  "authors": [
    {
      "name": "My name",
      "email": "My email",
      "role": "Maintainer"
    }
  ],
  "repositories": [
    {
      "type": "path",
      "url": "../../code"
    },
     {
      "type": "vcs",
      "url": "git@github.com:myname/magento-core-composer-installer.git"
    }
  ],
  "extra": {
    "magento-core-package-type": "magento-source",
    "magento-root-dir": "/var/www/html",
    "magento-deploystrategy": "symlink"
  },
  "minimum-stability": "dev",
  "prefer-stable": true
}

Package composer.json

{
  "name": "myproject/trt-source",
  "license": "OSL-3.0",
  "authors": [
    {
      "name": "My name",
      "email": "My email",
      "role": "Maintainer"
    }
  ]
}

I am using docker for windows and running composer in linux container.
Just to make sure it's not something windows related i checked files from inside container
test -h filename && echo "is symlink" || echo "is regular file"
and result is is regular file

When i manually create symlinks using ln -s /dir1/filename /dir2/filename they work as symlinks and result in is symlink on test above.

What can be the reason that "magento-deploystrategy": "symlink" is not interpreted correctly ?