[20.10 backport] replace json.Unmarshal with NewFromJSON in Create

[thaJeztah]

backport of #41701

fixes #41683 /image/store.go Empty configuration causes runtime panic

- What I did

Reuse NewFromJSON to handle no rootfs key situation in image/store.go to solve the issue in #41683

- How I did it

Simply replace the json.Unmarshal function with NewFromJSON in image/image.go

- How to verify it

Running this (mainly modified from #41683) will show invalid image JSON, no RootFS:

package main

import (
    "github.com/docker/docker/image"
    "os"
    "io/ioutil"
    "runtime"
    "github.com/docker/docker/layer"
    "fmt"
)


type mockLayerGetReleaser struct{}

func (ls *mockLayerGetReleaser) Get(layer.ChainID) (layer.Layer, error) {
        return nil, nil
}

func (ls *mockLayerGetReleaser) Release(layer.Layer) ([]layer.Metadata, error) {
        return nil, nil
}

func main(){
    tmpdir, err := ioutil.TempDir("", "images-fs-store")
    defer os.RemoveAll(tmpdir)
    if err != nil {
        fmt.Print(err)
        os.Exit(1)
    }
    fsBackend, err := image.NewFSStoreBackend(tmpdir)
    if err != nil {
        fmt.Print(err)
        os.Exit(1)
    }
    mlgrMap := make(map[string]image.LayerGetReleaser)
    mlgrMap[runtime.GOOS] = &mockLayerGetReleaser{}
    store, err := image.NewImageStore(fsBackend, mlgrMap)
    if err != nil {
        fmt.Print(err)
        os.Exit(1)
    }
    _, err = store.Create([]byte(`{}`))
    if err != nil {
        fmt.Print(err)
        os.Exit(1)
    }
}

- Description for the changelog

Prevent a panic when handling an image json with no `rootfs` key present.

[thaJeztah]

rebased to trigger CI with test-fixes that were merged

[cpuguy83]

LGTM