build --build-arg KEY with no explicit value considered an invalid value

[ghost]

Description

$ cat Dockerfile
FROM alpine

ARG HTTP_PROXY
$ nerdctl build -t test --build-arg HTTP_PROXY .
error: invalid opt: invalid value build-arg:HTTP_PROXY
FATA[0000] unrecognized image format
FATA[0000] exit status 1
$ nerdctl build -t test --build-arg HTTP_PROXY=foo .
[+] Building 0.8s (2/3)

That is, --build-arg HTTP_PROXY is considered invalid without an explicit value, but according to https://docs.docker.com/engine/reference/commandline/build/#set-build-time-variables---build-arg it should be valid.

Steps to reproduce the issue

No response

Describe the results you received and expected

Got an error, expected no error.

What version of nerdctl are you using?

Client:
 Version:	v0.20.0
 OS/Arch:	linux/amd64
 Git commit:	e77e05b5fd252274e3727e0439e9a2d45622ccb9

Server:
 containerd:
  Version:	v1.5.8
  GitCommit:	1e5ef943eb76627a6d3b6de8cd1ef6537f393a71

Are you using a variant of nerdctl? (e.g., Rancher Desktop)

Colima

Host information

Client:
 Namespace:	default
 Debug Mode:	false

Server:
 Server Version: v1.5.8
 Storage Driver: overlayfs
 Logging Driver: json-file
 Cgroup Driver: cgroupfs
 Cgroup Version: 1
 Plugins:
  Log: json-file
  Storage: native overlayfs
 Security Options:
  seccomp
   Profile: default
 Kernel Version: 5.10.109-0-virt
 Operating System: Alpine Linux v3.14
 OSType: linux
 Architecture: x86_64
 CPUs: 4
 Total Memory: 19.55GiB
 Name: colima
 ID: d89c87bd-8854-43c5-bb6d-d1bcaffd74b9

WARNING: bridge-nf-call-iptables is disabled
WARNING: bridge-nf-call-ip6tables is disabled