Docker OpenWRT SDK - config problem

For Developers
1

I am new to the OpenWRT world and have recently encountered a problem with building an image. This is powershell scrip I wrote:

docker run --name openwrt_sdk --rm -v ${pwd}/bin/:/builder/bin -i -d openwrt/sdk:rockchip-armv8-SNAPSHOT
docker cp input/config.buildinfo openwrt_sdk:/builder/.config
docker exec -u root openwrt_sdk chown buildbot /builder/.config
docker exec openwrt_sdk make defconfig
  1. I spawn openwrt/sdk container for rockchip
  2. I copy config.buildinfo to that container and rename it to .config
  3. After copy operation file is root owned, change it to buildbot used by container
  4. run make defconfig command

config.buildinfo have a lot of lines but for this example I just need one:

CONFIG_TARGET_ROOTFS_PARTSIZE=512

And now the problem. As I inspected file is copied correctly but after executing make defconfig file .config get replaced by autogenerated one but my config is ignored, so final config contains a lot of lines but PARTSIZE is still defautl one:

CONFIG_TARGET_ROOTFS_PARTSIZE=104

Why make defconfig doesnt work as I would expect and place 512 instead 104?