Add container image support

[JSCU-CNI]

This PR adds container image support to dissect in the form of a loader and filesystem. It implements the Docker image spec and OCI container image spec.

Example usage:

$ docker pull python:latest && docker image save python:latest -o image.tar
$ target-shell image.tar
debuerreotype:/$ ls
$fs$
bin
boot
dev
etc
home
lib
lib64
media
mnt
opt
proc
root
run
sbin
srv
sys
tmp
usr
var
debuerreotype:/$ python
In [1]: t.fs.mounts["/"]
Out[1]: <ContainerImageFilesystem path=/tmp/image.tar name=python:latest>

[Schamper]

I think this feature fits better when entirely implemented as a loader, rather than a loader and a separate filesystem. I don't think I see anything unique in the filesystem implementation that can't be handled in the loader, but correct me if I'm wrong.