container: root: add getter for dfi's root path

Adds impl/test for getting absolute path to all `dfi`'s `root` code.

container/src/root/common/utility.hh

@@ -255,6 +255,16 @@ std::string get_env(const std::string& var)
   return std::string{env};
 }
 
+//! \brief Get `dfi`'s root-related code directory
+//! \details Operating system path to where all `dfi` root-related code resides
+//! \return Operating system absolute path (with trailing slash)
+//! \exception type::RuntimeError if unable to get environment
+//! \since docker-finance 1.1.0
+std::string get_root_path()
+{
+  return get_env("DOCKER_FINANCE_CONTAINER_REPO") + "/src/root/";
+}
+
 //! \brief Execute a command in operating system shell
 //! \param cmd Operating system shell command [args]
 //! \return Return value of command