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Hi, yes you're right - you can reference an image ID. However, as soon as you go to the network you're lost - any apt-get/yum update or install could break your system in surprising ways.

Having done _lots_ of builds lately I can vouch for that (see my blog for some examples).

In the end the image ID _is_ useful, but the dockerfile itself has limitations.

I agree with your last point as well - my evangelism comes from solving problems at my company in this way (which I know are not uncommon problems) rather than any belief that it beats others objectively.