Support `--set` for groups

[cpuguy83]

If I call a group instead of a target, if I need to use --set I have to know everything in the group.
Instead it could be nice if we could use --set <group>.<thing> and have that propagate to all targets in a group.

Maybe this is pretty specific to how I'm using group currently.

[tonistiigi]

Would you also want to have all the target properties enabled for group or is this use-case limited to --set for now?

[cpuguy83]

Would you also want to have all the target properties enabled for group

I'm not sure I understand this.

[tonistiigi]

like

group "default" {
	targets = ["db", "webapp-dev"]
        platforms = ["linux/amd64", "linux/arm64"]
}

target "webapp-dev" {
...
}

[cpuguy83]

Ah yes definitely.