Allow cache probing to work when instructions (e.g. RUN) don't change any files

[mafredri]

Related: #230

In coder/kaniko#25 we added support for multi-stage cache probing. While testing the implementation we noticed that sometimes an instruction (e.g. RUN systemctl enable ssh) doesn't change any files. This means the snapshotter won't create a tar archive and the cache layer won't be pushed to the remote registry.

This is problematic when running the cache probe as these instructions will not have a matching cache.

We also can't assume that a missing cache is OK because the hash of the instruction does not affect the next instruction (bug?). Essentially the instruction could be modified (RUN systemctl enable ssh => RUN systemctl disable ssh) and we'd still have a cache hit if we assumed a missing cache was OK.

Potential solutions to fix the problem:

1. Push empty cache layers for instructions with no-change
2. Allow md5(RUN systemctl enable ssh) to be part of the next instructions cache key

[johnstcn]

cc @bpmct -- we may want to pull this into #132

[bpmct]

Agreed, let's do it. We also set Oct 1 as the deadline for v1.0 release