Follow-up to !1664. The --mtu 1360 workaround was documented only as a
YAML comment, which is invisible in pipeline logs. Emit a single-line
runtime warning so anyone reviewing build output knows a reduced MTU is
temporarily in effect and where to track its removal.

Relates https://gitlab.com/gitlab-com/gl-infra/production-engineering/-/work_items/28488

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

fix(docker): warn at build time that a reduced MTU workaround is in use

See merge request !1701

## [4.30.1](v4.30.0...v4.30.1) (2026-07-13)