Using Docker history 

We can check the history of image modifications using docker history:

$ docker history custom_image2
IMAGE CREATED CREATED BY SIZE COMMENT
6b9be8efcb3a 21 hours ago /bin/sh -c echo "This is a custom image" > /u 23 B
01154c38b473 2 weeks ago /bin/sh -c #(nop) CMD ["httpd-foreground"] 0 B
...
output truncated for brevity
...

Note that a new layer, 6b9be8efcb3a, is added. This is where we change the content of the index.html file in comparison to the original httpd image.