docker提交失败，返回“0”；没有这样的id“；

通过docker教程学习如何在mac上使用docker：

正在尝试更新和提交映像。
收到我不理解的错误消息

$docker pull training/sinatra
Pulling repository training/sinatra
...
79e6bf39f993: Download complete 
Status: Downloaded newer image for training/sinatra:latest

$docker run -t -i training/sinatra /bin/bash

root@6a2012d33405:/# gem install json
Fetching: json-1.8.2.gem (100%)
Building native extensions.  This could take a while...    
Successfully installed json-1.8.2
1 gem installed
Installing ri documentation for json-1.8.2...
Installing RDoc documentation for json-1.8.2...

root@6a2012d33405:/# exit
exit

$docker images
REPOSITORY          TAG                 IMAGE ID            CREATED             VIRTUAL SIZE
...
hello-world         latest              91c95931e552        6 weeks ago         910 B
training/sinatra    latest              f0f4ab557f95        11 months ago       447 MB

$docker commit -m "Added json gem" -a "Yours Truly" f0f4ab557f95 ouruser/sinatra:v2
FATA[0000] Error response from daemon: no such id: f0f4ab557f95 

---

看起来我使用了错误的id。
“图像ID”与“容器ID”不同。
后者是提示中“root@”之后出现的内容root@6a2012d33405“
这项工作：

$docker commit -m "Added json gem" -a "Yours Truly" 6a2012d33405  ouruser/sinatra:v2
36c544150c9405934674b52ffc447519cb428c0526607276daab56025dfd6b11

$docker images
REPOSITORY          TAG                 IMAGE ID            CREATED              VIRTUAL SIZE
ouruser/sinatra     v2                  36c544150c94        About a minute ago   452.1 MB
...
hello-world         latest              91c95931e552        6 weeks ago          910 B
training/sinatra    latest              f0f4ab557f95        11 months ago        447 MB