Support non-distributable layers in Write and MultiWrite

[StevenLocke]

Implements #741

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[jonjohnsonjr]

Neat, thanks! Once CLA is sorted out this looks pretty good.

I'll want to expose this option in pkg/crane and as a flag for --crane, but I can do that later myself.

[jonjohnsonjr]

This is a mouthful, but I don't have a better suggestion. Maybe WithNondistributable is good enough?

This could also take a bool if we want WithNondistributable(false) to work as an explicit "don't push these" option, to override any other options.

If that doesn't seem plausible, we can drop the thunk if this will only ever be true.

cc @imjasonh bikeshed naming here?

[imjasonh]

+1 to WithNonDistributable and a clear docstring. If we find a better alternative we can deprecate this and call it something else later.

Also +1 to making it only true-able, and not make people think about what WithNonDistributable(false) means. 😄

[jonjohnsonjr]

Also +1 to making it only true-able, and not make people think about what WithNonDistributable(false) means. 😄

I was suggesting the opposite actually, but I'm open to being convinced.

The point I was trying to make is that if we do decide only true-able, I'd rather see:

foo(bar, remote.WithNondistributable)

than

foo(bar, remote.WithNondistributable())

WithNonDistributable

I'm gonna be super pedantic here but I think lowercasing the D matches the media type better, if we go with this, so:

WithNondistributable is my preference.

[imjasonh]

I defer to Jon, remote.WithNondistributable (no ()) sgtm

[StevenLocke]

@googlebot I signed it!

[jonjohnsonjr]

LGTM, thanks!