Getting permission denied error when trying to run mix release

I followed this tutorial and created a build with few modifications. After generating a build with dockerfile, when I try to run the release I get permission denied error.

Context:

• Trying to generate release with docker file and put tar release back in _build folder so I can upload to server and run
• The build was successful and here is code, I tried

Steps to reproduce on sample project:

• git clone https://github.com/tanweerdev/build_permission
• cd into project
• mix deps.get
• mix docker.build prod
• _build/prod/rel/my_build/bin/my_build start_iex

and you will get permission denied error

BUT if you try with sudo, it works. I dont know why permissions gets changed. Any help why we need sudo? or is it a good solution to change permission after build is generated?