Adivice emacs exunit to use a custom mix command (e.g a docker container)

.dir-locals.el

((elixir-mode . ((my/use-exunit-custom . t)
                 (my/exunit-custom-mix-cmd . "docker exec -it <container_id_or_name> mix"))))

custom.el

;; advice exunit execution to use Doca
(defvar my/use-exunit-custom nil)
(defvar my/exunit-custom-mix-cmd "docker exec -it <container_id_or_name> mix")

(defun my/exunit-compile (orig-fun &rest args)
  (if my/use-exunit-custom
      (let* ((directory (cadr args))
             (default-directory (or directory (exunit-project-root)))
             (compilation-environment exunit-environment)
             (args (car args)))
        (exunit-do-compile
         (s-join " " (append (list my/exunit-custom-mix-cmd "test") exunit-mix-test-default-options args))))
    (apply orig-fun args)))

(advice-add 'exunit-compile :around #'my/exunit-compile)

;; to remove
;; (advice-remove 'exunit-compile #'my/exunit-compile)

Customize and move code from .dir-locals.el to root of your Elixir project.