Hello,
The final solution to this issue is here.
http://write.w5hnews.com/u/cwSNkeLNXf#fly-deploy-elixir-pheonix-app-with-npm
It consists of two parts that are not obvious from the error message.
1st: You need npm to install above assets. So they need to be added to docker. See the link above.
2nd: When you add that, fly.toml will fail because of during deployment it has migration script. (Which as a new project you might not have yet)
Enjoy. Solved.