Add iidfile flag for build image

[junnplus]

related #215

[ktock]

Got a different result than docker after some manual testing.

docker:

$ docker build -t test --iidfile=/tmp/iid /tmp/ctx
$ cat /tmp/iid
sha256:c74973c38ea79154adb9c8caf29e4be8f9b09690f08cd36bee480b3568ba2da5
$ docker image inspect test | jq -r '.[0].Id'
sha256:c74973c38ea79154adb9c8caf29e4be8f9b09690f08cd36bee480b3568ba2da5

nerdctl:

# nerdctl build -t test --iidfile=/tmp/iid /tmp/ctx
# cat /tmp/iid
sha256:84199e0a1de5a053170744bd4b7d6333480ada680136a9fed0e811e409fc1946
# nerdctl image inspect test | jq -r '.[0].Id'
sha256:c7a9088f63ec485c116a91e5f6834c2dba5e55da5a390b87a8864bcde9778432

iidfile should be that same as nerdctl image inspect at least.

[AkihiroSuda]

iidfile should be that same as nerdctl image inspect at least.

No.
#442

[ktock]

LGTM on green

[AkihiroSuda]

Added a follow-up commit for docs