Multi-stage app builds left an untagged image for every build stage after each deploy, and nothing could safely remove them. upaas now asks Docker for a BuildKit build, which keeps stages in Docker's build cache instead of as images; Docker limits and cleans that cache itself. The final image is still tagged `upaas-<app>:<N>`, so old-image cleanup is unchanged. BuildKit's progress messages are decoded and written to the deployment log as plain text.
Deviation: adds `github.com/moby/buildkit` v0.16.0 (matching the pinned Docker client), added with `go get` since no make target adds dependencies.
Judgement call: the cache limit is on by default only from Docker Engine 28.2; older engines need the `daemon.json` setting the README now names.
Model: opus-5-5
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