cg188: secret mounts

[mzihlmann]

Fixes https://github.com/chainguard-dev/kaniko/issues/188 GoogleContainerTools/kaniko#3028

Description

Adds support for secret mount syntax. Note that this does not store the secrets securely, any secrets passed to kaniko must be considered exposed.

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• Includes unit tests
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• The code flow looks good.
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Release Notes

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

Examples of user facing changes:
- kaniko adds a new flag `--registry-repo` to override registry

[mzihlmann]

I was having a think about how we expose the cli arg and I think docker is correct. We shouldnt push the anti-pattern of putting secrets onto the cli. As we're in k8s they anyways can only come in env variables or file, so the docker implementation is actually convenient.

[mzihlmann]

let me do the docker style cli arg in a second part as it would only explode the contents of this PR