func newUserDirective(base *baseDirective, state *parsingState) (Directive, error) {
	if err := base.replaceVarsCurrStage(state); err != nil {
		return nil, err
	}
	args := strings.Fields(base.Args)
	if len(args) != 1 {
		return nil, base.err(errNotExactlyOneArg)
	}
	return &UserDirective{base, args[0]}, nil
}
Variables:
Replaced from ARGs and ENVs from within our stage.
Formats:
USER <user>[:<group>]
package dockerfile
import (
	"strings"
)
type UserDirective struct {
	*baseDirective
	User string
}