docker image version rollback

If we want to make a rollback and remove the last layer (maybe the file should be called c.txt instead of b.txt) 
we can do 
so by tagging the layer 9977b78fbad7:
$ docker tag 9977b test/b
Let’s take a look at the new history:

$ docker history test/b
IMAGE               CREATED             CREATED BY                                      SIZE
9977b78fbad7        3 hours ago         /bin/sh -c apt-get install -y apache2           54.17 MB
e83b3bf07b42        3 hours ago         /bin/sh -c apt-get update                       20.67 MB
9cd978db300e        3 months ago        /bin/sh -c #(nop) ADD precise.tar.xz in /       204.4 MB
6170bb7b0ad1        3 months ago        /bin/sh -c #(nop) MAINTAINER Tianon Gravi <ad   0 B
511136ea3c5a        10 months ago                                                       0 B
Our last layer is gone and with the layer the text file b.txt!