TRAMPを使ってdockerコンテナ内のファイルを開く ref: http://qiita.com/osamu2001/items/3058f7c3e3d8d9a6e532

docker-tramp-methods.el

;; http://www.emacswiki.org/emacs/TrampAndDocker
(push
 (cons
  "docker"
  '((tramp-login-program "docker")
    (tramp-login-args (("exec" "-it") ("%h") ("/bin/bash")))
    (tramp-remote-shell "/bin/sh")
    (tramp-remote-shell-args ("-i") ("-c"))))
 tramp-methods)

(defadvice tramp-completion-handle-file-name-all-completions
    (around dotemacs-completion-docker activate)
  "(tramp-completion-handle-file-name-all-completions \"\" \"/docker:\" returns                                                                                              
    a list of active Docker container names, followed by colons."
  (if (equal (ad-get-arg 1) "/docker:")
      (let* ((dockernames-raw (shell-command-to-string "docker ps | perl -we 'use strict; $_ = <>; m/^(.*)NAMES/ or die; my $offset = length($1); while(<>) {substr($_, 0, $\
offset, q()); chomp; for(split m/\\W+/) {print qq($_:\n)} }'"))
             (dockernames (cl-remove-if-not
                           #'(lambda (dockerline) (string-match ":$" dockerline))
                           (split-string dockernames-raw "\n"))))
        (setq ad-return-value dockernames))
    ad-do-it))