Dockerfile Layer Audit

Audit one Dockerfile for what its layers will cost and what its build records. Line continuations are joined first, which is what makes the layer questions answerable at all. Reports a package index updated in a layer of its own, so a later install uses a cached index months stale; a dependency install after a copy of the whole context, so that layer rebuilds on every source change; a download piped into a shell; and a credential-shaped build variable, recorded in the image history.