PoC: Build a tool to validate file hashes against layer caches

[mafredri]

This issue tracks the implementation of a PoC to validate the path forward for #128.

To support #185, we must be able to figure out if a cached layer image is valid, given the state of files relevant to building the container (think contents of Dockerfile, devcontainer.json, and hashes of any files pulled in via COPY-directive).

In this PoC, we will implement this logic (files -> layer) as a subcommand of envbulider. This will require parsing and understanding of Dockerfile to understand if a change modifies the outcome, and which layers remain intact.