`stack image`: allow absolute source paths for `add`
`stack image`: allow absolute source paths for `add`

AndreasPK/stack,mrkkrp/stack,mrkkrp/stack,Heather/stack,juhp/stack,MichielDerhaeg/stack,Fuuzetsu/stack,phadej/stack,martin-kolinek/stack,deech/stack,martin-kolinek/stack,deech/stack,MichielDerhaeg/stack,AndrewRademacher/stack,Blaisorblade/stack,AndreasPK/stack,phadej/stack,AndrewRademacher/stack,anton-dessiatov/stack,anton-dessiatov/stack,Heather/stack,sjakobi/stack,sjakobi/stack,juhp/stack,Blaisorblade/stack,Fuuzetsu/stack