Add s390x kaniko build to multi-arch list

[barthy1]

Description

This is extension of current code to build s390x version of kaniko executor image and add it to multi-arch manifest.

Unfortunately at this moment integration tests cannot be added to this PR, as bazel binary is not publicly available for s390x. Tests on my local s390x environment using https://github.com/GoogleContainerTools/kaniko/blob/master/.travis.yml#L50-L57 are PASSED

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• Includes unit tests
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Release Notes

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The executor:multi-arch image now supports 3 architectures amd64, arm64 and s390x.

[tejal29]

@barthy1 would you to also add s390x to the debug image?

[barthy1]

@tejal29

would you to also add s390x to the debug image?

At this moment debug image is not multi-arch at all. I'd suggest to have separate PR for that.

[tejal29]

@tejal29 to follow up the s390x image from master for @barthy1 to test.

[tejal29]

@barthy1 Please verify if this image works as expected.
gcr.io/kaniko-project/executor:s390x-49e7d8a8cd2e0f071881afa8f3a9d3935e9f7103

[barthy1]

@tejal29 I verified that image is runnable on s390x architecture and also verified that executor binary works as expected.

[tejal29]

Thanks. The next release is scheduled before Thanks giving.