Build apps with BuildKit so build stages stop piling up (closes #220)

The classic builder left an untagged image for every step of every
stage of a multi-stage build. BuildKit keeps stages in Docker's build
cache, which Docker keeps under a size limit on its own. BuildKit
reports progress as encoded trace messages, so these are decoded with
BuildKit's own library and written to the deployment log as plain text.
The README notes that engines older than 28.2 need build cache cleanup
turned on in daemon.json.

Model: opus-5-5

TODO.md

@@ -20,6 +20,11 @@ regress.
 
 # Completed Steps
 
+- 2026-09-23: Apps are now built with BuildKit, so the stages of a multi-stage
+  build stay in Docker's size-limited build cache instead of piling up as
+  untagged images; build progress is still written to the deployment log as
+  plain text (#220).
+
 - 2026-09-23: After a successful deploy, upaas removes the app's images other
   than the running one and the one rollback would use, together with the
   untagged images they were built on (#216).