Bug#893037: Add support for diffing docker-format containers

[Jonathan Dowland]

I poked a bit at how docker's overlay2 driver stores the layers and came
up with this temporary hack script

> #!/bin/sh
> set -u
> set -e
> 
> if [ $# -lt 2 ]; then
>     echo usage: $0 before after
>     exit 1
> fi
> 
> before="$1"
> after="$2"
> 
> bfs=$(docker inspect -f '{{ .GraphDriver.Data.UpperDir }}' "$before")
> afs=$(docker inspect -f '{{ .GraphDriver.Data.UpperDir }}' "$after")
> 
> diff -ruN "$bfs" "$afs"

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⣾⠁⢠⠒⠀⣿⡁ Jonathan Dowland
⢿⡄⠘⠷⠚⠋⠀ https://jmtd.net
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