multi-arch builds

[mzihlmann]

Although kaniko cannot run non-native RUN statements, that doesn't mean kaniko can't produce images for non-native architecture. If all we do is COPY, ENV and the like, arch actually doesn't matter and we can "cross-compile" a dockerfile. You might think that this is not really helpful, but our own dockerfile here that we use to produce kaniko images for all different archs, actually would work (mostly) just fine with this limited supported for multi-arch builds. So there might be an actual use-case for this.

Implementation wise it shouldn't be too hard, the only problem I'm seeing is that this feature might be too confusing to users. People that are not aware of how kaniko works under the hood, might also not understand how multi-arch will be limited to not be able to run RUN statements and we might see an increase in support tickets as a result.