bake: fix missing default target in group's default targets

[crazy-max]

fixes #2859

77b3326 made special default target not being added to targets of the default group but it should otherwise it doesn't correlate with solved targets. But it should not add a default group to it. Also fix sorting to make output consistent.

cc @klutchell

Before

target "default" {
  dockerfile = "Dockerfile"
  platforms = [
    "linux/amd64"
  ]
}

target "multiarch" {
  dockerfile = "Dockerfile"
  platforms = [
    "linux/amd64",
    "linux/arm64",
    "linux/arm/v7",
    "linux/arm/v6"
  ]
}

$ docker buildx bake --print default multiarch
{
  "group": {
    "default": {
      "targets": [
        "multiarch"
      ]
    }
  },
  "target": {
    "default": {
      "context": ".",
      "dockerfile": "Dockerfile",
      "platforms": [
        "linux/amd64"
      ]
    },
    "multiarch": {
      "context": ".",
      "dockerfile": "Dockerfile",
      "platforms": [
        "linux/amd64",
        "linux/arm64",
        "linux/arm/v7",
        "linux/arm/v6"
      ]
    }
  }
}

group "default" {
        targets = ["app", "multiarch"]
}
target "app" {
        dockerfile = "app.Dockerfile"
}
target "foo" {
        dockerfile = "foo.Dockerfile"
}
target "multiarch" {
        dockerfile = "Dockerfile"
        platforms = ["linux/amd64","linux/arm64","linux/arm/v7","linux/arm/v6"]
}

$ docker buildx bake --print default foo
{
  "group": {
    "default": {
      "targets": [
        "app",
        "multiarch",
        "foo"
      ]
    }
  },
  "target": {
    "app": {
      "context": ".",
      "dockerfile": "app.Dockerfile"
    },
    "foo": {
      "context": ".",
      "dockerfile": "foo.Dockerfile"
    },
    "multiarch": {
      "context": ".",
      "dockerfile": "Dockerfile",
      "platforms": [
        "linux/amd64",
        "linux/arm64",
        "linux/arm/v7",
        "linux/arm/v6"
      ]
    }
  }
}

After

target "default" {
  dockerfile = "Dockerfile"
  platforms = [
    "linux/amd64"
  ]
}

target "multiarch" {
  dockerfile = "Dockerfile"
  platforms = [
    "linux/amd64",
    "linux/arm64",
    "linux/arm/v7",
    "linux/arm/v6"
  ]
}

$ docker buildx bake --print default multiarch
{
  "group": {
    "default": {
      "targets": [
        "default",
        "multiarch"
      ]
    }
  },
  "target": {
    "default": {
      "context": ".",
      "dockerfile": "Dockerfile",
      "platforms": [
        "linux/amd64"
      ]
    },
    "multiarch": {
      "context": ".",
      "dockerfile": "Dockerfile",
      "platforms": [
        "linux/amd64",
        "linux/arm64",
        "linux/arm/v7",
        "linux/arm/v6"
      ]
    }
  }
}

group "default" {
        targets = ["app", "multiarch"]
}
target "app" {
        dockerfile = "app.Dockerfile"
}
target "foo" {
        dockerfile = "foo.Dockerfile"
}
target "multiarch" {
        dockerfile = "Dockerfile"
        platforms = ["linux/amd64","linux/arm64","linux/arm/v7","linux/arm/v6"]
}

$ docker buildx bake --print default foo
{
  "group": {
    "default": {
      "targets": [
        "app",
        "foo",
        "multiarch"
      ]
    }
  },
  "target": {
    "app": {
      "context": ".",
      "dockerfile": "app.Dockerfile"
    },
    "foo": {
      "context": ".",
      "dockerfile": "foo.Dockerfile"
    },
    "multiarch": {
      "context": ".",
      "dockerfile": "Dockerfile",
      "platforms": [
        "linux/amd64",
        "linux/arm64",
        "linux/arm/v7",
        "linux/arm/v6"
      ]
    }
  }
}

[klutchell]

Thanks for looking into this so quickly!