Support image manifest lists / index

[schu]

Manifest lists application/vnd.docker.distribution.manifest.list.v2+json (or image indexes, application/vnd.oci.image.index.v1+json) are not yet supported and docker2aci throws an error in getManifestV21() if those are used. Example:

docker2aci docker://alpine@sha256:f006ecbb824d87947d0b51ab8488634bf69fe4094959d935c0c103f4820a417d
Error: conversion error: name doesn't match what was requested, expected: library/alpine, downloaded: