coder/kaniko: support multi-stage builds with DoCacheProbe

[johnstcn]

coder/kaniko#12 added support for performing a 'fake' build (e.g. ProbeCache)

However, this does not appear to work with multi-stage builds (coder/kaniko#12 (comment))

Add support for multi-stage builds. This will require changes to our Kaniko fork.

[mafredri]

To support multi-stage cache probes, we'll need a way to deduce the hash of dependencies between stages when operating in cache probe-mode.

Some ideas off the top of my head:

1. Use a tar reader/filesystem on the previous stage cache archive
   • Kaniko uses os and reads files directly, this will fail unless the previous stage has been extracted
   • Potentially slow, may require decompression and reading of the entire archive
   • Faster than normal extraction, small files are part of the tar stream and don't add overhead
2. Allow cached stage to be extracted
   • Slow, potentially very little benefit in cache probing
3. Modify cache archive/image to include metadata (i.e. file hashes of cross-stage dependencies)?
   • Probably doable, we can pre-compute cross-stage dependencies
   • Probably jank
4. Through some magic, allow the stage archive/hash to represent the cache-ability of the directive

I'm open to better ideas here.