foreach + subtask breaks the templating engine

[loopfz]

• I'm submitting a ...

  • bug report
  • feature request

• What is the current behavior?
  https://github.com/ovh/utask/blob/master/pkg/plugins/builtin/subtask/subtask.go#L43
  The subtask plugin uses an internal context object to exchange data between the main controller goroutine and itself. The templating instructions are generated in a Sprintf here, and results in e.g. {{.step.foobar.output}}.
  In the case of a foreach child step, the generated template instruction becomes {{.step.foobar-0.output}}, which makes the templating engine very unhappy because of the presence of a hyphen: failed to template context: Templating error: template: tmpl:1: bad character U+002D '-'.
  This is a common issue with texttemplate, see Go Templates and Parsing Errors When Hyphen is Used moby/moby#35886 Accessing values of the subchart with dash in the name helm/helm#2192 "Bad character" for valid YAML keys gohugoio/hugo#1474 etc.

• What is the expected behavior?
  The templating generation should be more cautious to avoid generating invalid templating instructions in that case.

• Which version of uTask are you using?
  1.1

• Please tell us about your environment

• Other information (e.g. detailed explanation, stacktraces, related issues, suggestions how to fix, links for us to have context, eg. stackoverflow, gitter, etc)