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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