Add useCurrentTimestamp flag for Jib builds

[twobiers]

Description

By default Jib will build with a timestamp set to Unix epoch, see: https://github.com/GoogleContainerTools/jib/blob/master/docs/faq.md#why-is-my-image-created-48-years-ago. However, the Quarkus extension sets the build time by default to the current timestamp, which can be counterproductive if you plan to ship reproducable builds.

Implementation ideas

A simple configuration flag like useCurrentTimestamp which defaults to true to not break the current behaviour would do its job I think.

[quarkus-bot]

/cc @geoand