Fail the sbt warm layer when both attempts fail

The `[ -n "$ok" ]` check was a no-op: errexit does not apply inside
the for loop, which is a non-final operand of the RUN's `&&` list, so
the loop still exited 0 after two failed attempts. Exit explicitly.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_011f9aePV6aRxWWuVgK81qyD

Author: claude

tests/docker/Dockerfile.sbt

@@ -121,7 +121,7 @@ RUN set -eu \
         fi; \
         echo "warm sbt $v update failed or ran past 600 s (attempt $attempt)" >&2; \
       done; \
-      [ -n "$ok" ]; \
+      [ -n "$ok" ] || exit 1; \
       rm -rf "$d"; \
     done \
  && if [ "$SBT_WARM_TOOLS" = 1 ]; then \