Use EPOCH timestamp as a file modification timestamp when adding libs docker layer

[fbotis]

Use Instant.EPOCH when adding libsMainPath layers so if the content of the files do not change the layer digest doesn't change either

Fixes: 26734

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[geoand]

Thanks for this!

I don't think we want to limit to only that layer - I think the setting should be for all layers

[geoand]

Thanks for the update.

I added a comment about the documentation.

We'll also need the commits squashed

[geoand]

I think we should also explain what this does, basically what the effect of turning it off means for layering.

[fbotis]

Appending something like the paragraph below?

If the modification time is constant (flag is set to false so Unix epoch is used) across two consecutive builds, the docker layer sha256 digest will be different only if the actual files added by Jib to the docker layer were changed. More exactly, having 2 consecutive builds will generate different docker layers only if the actual content of the files within the docker layer was changed.

[geoand]

👍🏼

[gsmet]

Superseded by #26758 .