config language: add Starlark go config language

[sgotti]

https://github.com/google/starlark-go/blob/master/doc/spec.md

It should satisfy our reproducibility needs (like jsonnet) and could be easier to write agola config files.

Like in #137 we should provide build context information.

[tboerger]

Maybe https://github.com/drone/drone-convert-starlark could be useful to get an idea how other projects integrate Starlark. I really like that.

[camandel]

Maybe https://github.com/drone/drone-convert-starlark could be useful to get an idea how other projects integrate Starlark. I really like that.

I think starlark config file support has been added with commit d91ec09 (but not yet included into docker image, you need to compile it from sources).
Tried with a simple config.star file and it works:

def main(ctx):
   return run("my run", "my task")


def run(runname, taskname):
  return {
    "version": "v0",
    "runs": [
      {
        "name": runname,
        "tasks": [
           {
            "name": taskname,
            "runtime":
             {
               "type": "pod",
               "arch": "amd64",
               "containers":
                 [
                   { "image": "alpine:latest" }
                 ]
             },
            "steps": steps(3)
           }
        ]
      }
    ]
  }

def steps(n):
  steps = []

  for i in range(n):
    cmd =  { "run": "/bin/sleep %d" % (i + 1) }
    steps.append(cmd)

  return steps