warn on duplicate cache-to targets

[areebahmeddd]

summary

closes #3819

• adds a warning when multiple build targets try to use the same cache destination

ai model disclosure

used vscode copilot (claude sonnet 4.6) to understand the issue context and help generate a fix. changes were self-reviewed and verified via:

make validate-all
go build ./...

[areebahmeddd]

weird. the dco is failing although the commits are signed 🤔 let me try something.

[areebahmeddd]

ping! @crazy-max

[tonistiigi]

If added, this should be based on some primary values, eg. type=local,dest=, type=registry,ref=. Eg. if same ref is used with different mode then it is still a collision. And we shouldn't use the potentially secret values for comparison or log them anyway.

Does this work with target inheritance?

[areebahmeddd]

we shouldn't use the potentially secret values for comparison or log them anyway.

yea sure, i think using a helper func here is enough for comparing the destination key. creds/mode are ignored

Does this work with target inheritance?

yes, this runs after inheritance gets resolved so shared inherited cache-to values still trigger the warning

[areebahmeddd]

gentle ping! @tonistiigi

the ci failure is not related to our changes.