.git is excluded from the Docker build context (.dockerignore), so
@sentry/nextjs can't auto-detect a release via git and falls back to the
literal string "undefined" for --release, which made the sourcemaps
upload command fail with exit code 1 (confirmed via verbose build log).
start.sh now resolves the short SHA on the host and passes it through
as a SENTRY_RELEASE build arg, same pattern as the other Sentry vars.
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