Comment by stevenhuang

Those intermediate layers are usually part of the artifact. Try exporting an image with docker save and investigate what’s inside. This is all documented in a mostly comprehensible manner in the OCI specs.

I’m afraid you’re missing my point, though. A high quality build system takes fixed inputs and produces outputs that are, to the extent possible, only a function of the inputs. If there’s a separate process that downloads the inputs (and preferably makes sure they are bitwise identical to what is expected), fine, but that step should be strictly outside the inputs to the actual thing that produces the release artifact. Think of it as:

    artifact = build_process(inputs)

    inputs = fetch(credentials, cache, hashes, etc)

Or, even better perhaps:

    inputs = …
    assert hash(inputs) == expected

(And now, unless you accidentally hash your credentials into the expected hash, you can’t leak credentials into the output!)

Once you have commingled it so that it looks like:

    final output, intermediate layers = monolithic_mess(credentials, cache, etc)

Then you completely lose track of which parts are deterministic, what lives in the intermediate layers, where the credentials go, etc.

Docker build is not a good build system, and it strongly encourages users to do this the wrong way, and there are many, many things wrong with it, and only one of those things is that the intermediate layers that you might think of as a cache are also exposed as part of the output.