fix: source maps — ship pre-built artifact with injected debug IDs to Docker

The docker-ui job was rebuilding the UI from scratch inside Docker, producing
chunk hashes and JS files that didn't match the source maps uploaded to
GlitchTip, causing all stack traces to appear minified.

Fix:
- ui/Dockerfile: add PREBUILT=1 ARG; skip npm run build when set
- release.yaml upload-sourcemaps: re-upload artifact after sentry-cli inject
- release.yaml docker-ui: download injected artifact into ui/build/ and pass
  PREBUILT=1 so Docker reuses the exact same JS files whose debug IDs are in
  GlitchTip

ui/Dockerfile

@@ -21,7 +21,11 @@ ENV PUBLIC_BUILD_VERSION=$BUILD_VERSION
 ENV PUBLIC_BUILD_COMMIT=$BUILD_COMMIT
 ENV PUBLIC_BUILD_TIME=$BUILD_TIME
 
-RUN npm run build
+# PREBUILT=1 skips npm run build — used in CI when the build/ directory has
+# already been compiled (and debug IDs injected) by a prior job.  The caller
+# must copy the pre-built build/ into the Docker context before building.
+ARG PREBUILT=0
+RUN [ "$PREBUILT" = "1" ] || npm run build
 
 # ── Runtime image ──────────────────────────────────────────────────────────────
 # adapter-node bundles most server-side code, but packages with dynamic