add --timestamp option for build sub-command

[gzm55]

For producing deterministic image, we need pass a timestamp to buildah commit command. This timestamp should be passed from ansible-bender build ... or via the playbook variables. For building a squashed image in ci, we could use the commit timestamp as the value of this option.

[TomasTomecek]

Hi, thanks for opening this issue. This is the first time I learnt about buildah commit --timestamp.

I'm wondering if the value should be auto-populated by default and configurable. What would be your preference?

[gzm55]

if do not pass timestamp to buildah commit, it is auto-populated by buildah. So the new optional argument (as cli parameter of yaml parameter) can be easily transmitted to the underlying container engine.

[TomasTomecek]

oki, understood

I'll try to implement passing that argument then