[REQ] Allow targets section to reference built-in frontend

[cpuguy83]

What kind of request is this?

None

What is your request or suggestion?

We currently have a targets section like so:

targets:
  azlinux3:
    // target-specific stuff

We can also add a custom target and reference an external frontend to handle building it:

targets:
  custom:
    frontend:
      image: my.image:latest

But we can't create a custom target and tell it to use the built-in implementation.
This would be useful when you need to use the same underlying builder implementation but have some slight variances under a different target name

targets:
  windowscross:
    // target-specific stuff
  windows-host-process:
    builder: windowscross
    image:
      base: <custom base>

Or probably more accurately:

targets:
  windows:
    builder: windowscross
  windows-host-process:
    builder: windowscross
    image:
      base: <custom base>

Alternatively this could get added to the frontend section

targets:
  windows-host-process:
    frontend:
      internal: windowscross

Are you willing to submit PRs to contribute to this feature request?

• Yes, I am willing to implement it.

[cpuguy83]

It would be nice if this wasn't tied to the specific dalec implementation, so "internal" may not be best here.