> the easiest way would be to run it as a Docker container

Regarding this part, you can always just run a base image and add the app yourself. I'm on mobile so can't test, but should be along these lines:

    docker run --rm --name dockerfmt \
    -v /path/to/Dockerfile:/tmp/Dockerfile \
    golang:1.24-alpine sh -c \
    "apk add git && go run github.com/reteps/dockerfmt@latest /tmp/Dockerfile"

> against an existing file

For this part yes, you'd still need one, but it can be any of your own.