bake --print show wrong docker-bake.json

[eitsupi]

The function to display groups added in #720 does not seem to be working correctly.

The json shown in the following example will not work properly and an error will be displayed.

buildx/docs/reference/buildx_bake.md

Lines 101 to 131 in bcfd434

	```console
	$ docker buildx bake "git://github.com/docker/cli#master" --print
	#1 [internal] load git source git://github.com/docker/cli#master
	#1 0.686 2776a6d694f988c0c1df61cad4bfac0f54e481c8 refs/heads/master
	#1 CACHED
	{
	"group": {
	"default": [
	"binary"
	]
	},
	"target": {
	"binary": {
	"context": "git://github.com/docker/cli#master",
	"dockerfile": "Dockerfile",
	"args": {
	"BASE_VARIANT": "alpine",
	"GO_STRIP": "",
	"VERSION": ""
	},
	"target": "binary",
	"platforms": [
	"local"
	],
	"output": [
	"build"
	]
	}
	}
	}
	```

{
  "group": {
    "default": [
      "binary"
    ]
  },
  "target": {
    "binary": {
      "context": "git://github.com/docker/cli#master",
      "dockerfile": "Dockerfile",
      "args": {
        "BASE_VARIANT": "alpine",
        "GO_STRIP": "",
        "VERSION": ""
      },
      "target": "binary",
      "platforms": [
        "local"
      ],
      "output": [
        "build"
      ]
    }
  }
}

$ docker buildx bake -f docker-bake.json --print
[+] Building 0.0s (0/0)
docker-bake.json:4
--------------------
   2 |       "group": {
   3 |         "default": [
   4 | >>>       "binary"
   5 |         ]
   6 |       },
--------------------
error: docker-bake.json:4,7-15: Incorrect JSON value type; Either a JSON object or JSON array of objects is required here, to define arguments and child blocks., and 2 other diagnostic(s)

Isn't the correct answer as follows?

{
  "group": {
    "default": [
      {
        "targets": [
          "binary"
        ]
      }
    ]
  },
  "target": {
    "binary": {
      "context": "git://github.com/docker/cli#master",
      "dockerfile": "Dockerfile",
      "args": {
        "BASE_VARIANT": "alpine",
        "GO_STRIP": "",
        "VERSION": ""
      },
      "target": "binary",
      "platforms": [
        "local"
      ],
      "output": [
        "build"
      ]
    }
  }
}

Also, the contents of groups displayed when groups is specified seems to be wrong.
If we specify the above json and let it bake --print again, we will see the json with the default targets as default as shown below.

{
  "group": {
    "default": [
      "default"
    ]
  },
  "target": {
    "binary": {
      "context": "git://github.com/docker/cli#master",
      "dockerfile": "Dockerfile",
      "args": {
        "BASE_VARIANT": "alpine",
        "GO_STRIP": "",
        "VERSION": ""
      },
      "target": "binary",
      "platforms": [
        "local"
      ],
      "output": [
        "build"
      ]
    }
  }
}

My local environment is not yet buildx 0.7.0, so I haven't checked it at hand, but I found this behavior on GitHubActions runner.
https://github.com/rocker-org/rocker-versioned2/runs/4277390774?check_suite_focus=true#step:3:5813
(CI to check if docker-bake.json is in the correct format by executing the bake --print command)