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.