support RFC-3339 date format for OCI

Following OCI standards for containers, creation date is to be added following RFC-3339 standards.

A native way to generate this with kaniko is to use a --build-arg CREATED=$(date --rfc-3339=seconds). Unfortunately, your build image does not support the same arguments to date.

Could this be updated?

Current behavior:

docker run -ti --entrypoint="" gitlab-registry.cern.ch/ci-tools/docker-image-builder busybox date --rfc-3339=seconds
date: unrecognized option `--rfc-3339=seconds'
BusyBox v1.31.1 (2020-06-02 02:36:36 UTC) multi-call binary.