create templates for working on codeforces, and update dockerfile for g++

Author: Joshua Goller

Dockerfile

@@ -6,6 +6,8 @@ RUN apt-get -qq install -y \
   cdecl \
   clang \
   clang-tools \
+  g++ \
+  gcc \
   git \
   lldb \
   make \

codeforces/templates/Makefile

@@ -0,0 +1,3 @@
+include ../../makefiles/settings.mk
+include ../../makefiles/cpp-competitive-compile.mk
+include ../../makefiles/build.mk

codeforces/templates/solution.cpp

@@ -0,0 +1,8 @@
+#include <iostream>
+
+using namespace std;
+
+int main() {
+  cout << "Hello, codeforces!"
+       << "\n";
+}