The sygnal container images (since v0.16.0) hardcode a HEALTHCHECK which
relies on curl, but the image does not include curl, so the healthcheck
can never pass. See https://github.com/element-hq/sygnal/issues/326

This unblocks upgrading sygnal past v0.15.1.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>