Specify --custom-platform when building non-host platform images

For reasons that are a bit unclear, Kaniko now rejects some (but,
strangely, not all) images whose platforms don't match that of the host
machine. This appears to be the (allegely fixed) issue reported in [1].

Since Docker is awful and life is short, here we use the workaround
proposed in the ticket, using `--custom-platform` to override the image
platform, coercing Kaniko to do as we say.

[1] https://github.com/GoogleContainerTools/kaniko/issues/1995