Multistage ONBUILD COPY Support

[dani29]

Fixes #533.

Description

ONBUILD did not work correctly, because the resolveOnBuild() method occurred after the resolveStages() method was called, so commands like COPY --from=builder were not properly replaced with --from=0. Moreover, because the CalculateDependency() method were unaware of these dependencies, files copied onbuild weren't even registered to be kept beyond the stage they were used it.

I had to break down dockerfile.Stages(opts), which was a huge method, into smaller chunks doing the following:

1. parsing the dockerfile into []instructions.Stage (no changes here)
2. resolving instructions within stages with references to previous stages (keeping a mapping of stageName:stageIndex)
3. building KanikoStages from the []instructions.Stage (no changes here)

afterwards, adding the following:

• passing the map from step 2 to CalculateDependency() so it would register dependencies for ONBUILD files
• passing the map from step 2 to NewStageBuilder -> resolveOnBuild(), so the COPY commands "From" will be replaced with the stage index.

Submitter Checklist

These are the criteria that every PR should meet, please check them off as you
review them:

• Includes unit tests
• Adds integration tests if needed.

See the contribution guide for more details.

Reviewer Notes

• The code flow looks good.
• Unit tests and or integration tests added.

Release Notes

Describe any changes here so maintainer can include it in the release notes, or delete this block.

Examples of user facing changes:
- Support ONBUILD Copy in multistage builds (#533)

[tejal29]

@dani29 the code flows looks good.
Thanks for the refactoring!

[tejal29]

Please make this a local variable so only the tests in build package can override this.

[dani29]

👍

[tejal29]

LGTM, except 1 nit.
Also can you please mention, did you test this PR on the Dockerfile in #533

[dani29]

Yes, I've verified that the Dockerfile provided by @mbfisher in #533 can be built.
The other docker file did not have the same issue with ONBUILD.

[dani29]

@JordanGoasdoue, I've had to manually fix conflicts since git wasn't able to digest the changes of both this PR and #1160.
Any chance you could take a quick look and make sure I mess up anything with your flow?

[JordanGoasdoue]

Done @dani29 , it's all good

[kirkvogen]

Thank you for making this fix. I’ve been playing around with Kaniko and ran into this issue today. And then I do a search and find that you fixed it 15 minutes ago. Serendipity!

[kirkvogen]

I tried the latest releases of the image. It fixed the issue that I was seeing with multi-stage ONBUILD. Thanks!