1. Set PHOENIX_DOCKER_GENERATOR to the path of this folder, for example if it's at $HOME/repos/phx-docker-gen, then put this in your .{z,bash}rc file: export PHOENIX_DOCKER_GENERATOR=$HOME/repos/phx-docker
2. Then stick the file gen-phx in your path.
3. Run gen-phx appname; appname should be underscore-case format. The script changes "hostname" from "localhost" to "db" in config/dev.exs; it needs to match the database service named in docker-compose.yml, in this case it is db.

For an existing project, follow steps 1 and 2 above and run gen-phx-existing inside the project folder. It will add the 3 Docker files (or overwrite them if they exist, adjust the script per your needs), set up the containers and run migrations.

https://96codes.dev/elixir-phoenix-development-using-docker-and-docker-compose

https://dev.to/hlappa/development-environment-for-elixir-phoenix-with-docker-and-docker-compose-2g17

Different than the above 2: https://medium.com/swlh/use-docker-to-create-an-elixir-phoenix-development-environment-e1a81def1d2e

See this for options to pass to mix phx.new: https://hexdocs.pm/phoenix/Mix.Tasks.Phx.New.html