This is the final part of an optimization that I've been refactoring towards for awhile.
If the Dockerfile consists of no RUN commands, or cached RUN commands, followed by metadata-only
operations, we can skip downloading and unpacking the base image.
This change fixes that by properly "replaying" the Dockerfile and mutating the config when
calculating cache keys. Previously we were looking at the wrong cache key for each command
when there was more than one.
* Rework cache key generation a bit.
Cache keys are now based on the previous commands, rather than the previous state
of the filesystem.
* Refactor command interface a bit, only cache the context for commands that use it.
dlorenc