Support parsing json config with hcl v2

[vanstee]

Providing a docker-bake.json to docker buildx bake fails to parse because we were always assuming hcl formatted config after the hcl v2 upgrade.

$ cat <<'EOF' > docker-bake.json
{
  "variable": {
    "TAG": {
      "default": "latest"
    }
  },
  "group": {
    "default": {
      "targets": ["webapp"]
    }
  },
  "target": {
    "webapp": {
      "args": {
        "buildno": "${add(123, 1)}"
      }
    }
  }
}
EOF
$ docker buildx bake -f docker-bake.json --print webapp                                                                   
bake.json:1,1-2: Argument or block definition required; An argument or block definition is required here.  

Fixed this by using the json parser based on the file suffix.

$ docker buildx bake -f docker-bake.json --print webapp                                                                   
{                                                                                                                  
   "target": {                                                                                                     
      "webapp": {                                                                                                  
         "context": ".",                                                                                           
         "dockerfile": "Dockerfile",                                                                               
         "args": {                                                                                                 
            "buildno": "124"                                                                                       
         }                                                                                                         
      }                                                                                                            
   }                                                                                                               
}

Also fixes json keys for groups and targets.

Fixes #278

[tonistiigi]

Maybe just fallback on error instead of checking the suffix?

[tonistiigi]

@vanstee Also, could you add a test for this?

[vanstee]

That could work, but I'm worried we'll be losing out on good error info depending on the default and fallback ordering. For instance, it would be confusing to get errors for parsing an hcl config when you missed a comma in a json config. Let me know what you think; the fallback would be pretty easy to implement.

[tonistiigi]

@vanstee I guess if the file has json suffix we could just go to json directly. Otherwise, try both and fail with HCL errors. That would be similar to compose detection then https://github.com/docker/buildx/blob/master/bake/bake.go#L54-L70

[vanstee]

Nice I like it. Will do.

[aloysbaillet]

Hi!
I was updating our CI system to pickup the latest version of buildx but we're still using .json and I have this exact problem. I'm in no rush to upgrade, but I'd like to know if you're planning on a 4.0.2 version release with this fix?
Cheers,
Aloys