PoC: Modify envbuilder to support build resumption from any layer

[mafredri]

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

To better utilize the envbuilder cache, we want to extend envbuilder with support for build resumption from any previous layer so that the container runtime layer caching can be utilized to avoid file extraction overhead.

To put simply, if currently we do:

docker run -it --rm ghcr.io/coder/envbuilder:0.2.9
# build layer 1
# push ghcr.io/myorg/envbuilder-cache:aaa
# build layer 2
# push ghcr.io/myorg/envbuilder-cache:bbb
# build layer 3
# push ghcr.io/myorg/envbuilder-cache:ccc
# start

We want to be able to do this instead:

docker run -it --rm ghcr.io/myorg/envbuilder-cache:bbb
# resuming build from layer 2
# build new layer 3
# push ghcr.io/myorg/envbuilder-cache:ddd
# start

Note: In isolation, this may not improve performance. The container runtime should have previously (extracted) some or all of the cached layer that we're resuming from.