I don't think the author is talking about loading secrets from the environment - I think they're specifically talking about hardcoding secrets into the Dockerfi

I don't think the author is talking about loading secrets from the environment - I think they're specifically talking about hardcoding secrets into the Dockerfile and using the Dockerfile ENV directive to set secrets for the processes running in the container (baking them into the image), instead of passing them at runtime, which sounds just horrifying enough that I'm sure people do it in real codebases.