Print simply STL containers of containers recursively in C++

//I have used the following code which work well for printing a simple std::vector with printContainer().

I want now to extend it for nested containers with printContainerV2()

I have tried using template to determine if the type is a stl container but it does not seem to be the way to do it.

#include <iostream>
#include <iterator>
#include <vector>

template <typename Iter, typename Cont>
bool isLast(Iter iter, const Cont& cont)
{
    return (iter != cont.end()) && (next(iter) == cont.end());
}


template <typename T>
struct is_cont {
    static const bool value = false;
};

template <typename T,typename Alloc>
struct is_cont<std::vector<T,Alloc> > {
    static const bool value = true;
};


template <typename T>
std::string printContainer(T const& container)
{
    std::string str = "{";
    for (auto it = std::begin(container); it != std::end(container); ++ it)
        if (isLast(it, container))
                str = str + std::to_string(*it) + "}";
        else
                str = str + std::to_string(*it) + ",";
    return str;
}
/*
template <typename T>
std::string printContainerV2(T const& container)
{
    std::string str = "{";
    for (auto it = std::begin(container); it != std::end(container); ++ it)
        if (isLast(it, container))
            if (is_cont<decltype(*it)>::value == true)
                str = str + printContainer(*it);
            else
                str = str + std::to_string(*it) + "}";
        else
            if (is_cont<decltype(*it))>::value == true)
                str = str + printContainer(*it);
            else
                str = str + std::to_string(*it) + ",";
    return str;
}
*/
int main()
{
    std::vector<int> A({2,3,6,8});
    std::vector<std::vector<int>> M(2,A);
    M[1][0] ++;

    std::cout << is_cont<decltype(A)>::value << std::endl;  // returns true !

    for (auto it = std::begin(M); it != std::end(M); ++ it)
    {
        std::cout << printContainer(*it) << std::endl; // works well std::vector<int>
        std::cout << is_cont<decltype(*it)>::value << std::endl; // return false :(
    }

    // Want to use this for std::vector<std::vector<int>>
   //  std::cout << printContainerV2(M) << std::endl; // not working !

}