update: app-containers/docker-buildx

[tormath1]

Name: app-containers/docker-buildx CVEs: CVE-2025-0495 CVSSs: 4.1 Action Needed: upgrade to >= 0.21.3

Summary: Some cache backends allow configuring their credentials by setting secrets directly as attribute values in cache-to/cache-from configuration. If this was done by the user, these secure values could be captured together with OpenTelemetry trace as part of the arguments and flags for the traced CLI command. Passing tokens to Github cache backend via environment variables or using registry authentication is not affected.

workaround: Avoid passing cache backend credentials with CLI arguments. Make sure access to traces and BuildKit history records is kept secure.

refmap.gentoo: TBD