proposal: missing target resolution for bake

When buildx bake is called with target names that don't exist, instead of erroring, we could allow a better default behavior.

The default behavior could be that in that case, we assume implicit target definition:

target "<name>" {
  target: "<name>"
}

That means that we will build the target from Dockerfile with the same name. As this only works with default Dockerfile properties anyway, we could first check if the Dockerfile locally exists and return a quick error otherwise.

To give users more control over this behavior we could also define a special target name that gets called in this case. Eg.

target "_bake_target_missing" {

target "*" {

If such target is set then it would be built instead of the default behavior for names that don't have a direct match.

Inside this target, the user can use two additional variables BAKE_CURRENT_TARGET and BAKE_ALL_TARGETS . So for example they can define that default behavior for bake foo bar is to build a specific Dockerfile target and pass foo bar as a build argument.

target "_bake_target_missing" {
  args = {
    "PACKAGES": BAKE_ALL_TARGETS
  }
}

@tiborvass @crazy-max

The issue names no files, tests, or entry points. Start by locating buildx bake's target-resolution flow and review how missing targets are currently handled; done requires an agreed behavior for implicit targets or a fallback target, including the proposed variables and error handling.