Add valgrind Memory Leak Checks

[etscrivner]

Add the ability to run Valgrind's memcheck to check for memory leaks in when running the test suite.

Adds the following three new make targets:

• make docker - creates a Docker image using the new Dockerfile.
• make memcheck - runs valgrind memcheck against the rspec test suite.
• make memcheck-docker - creates Docker container and runs memcheck within it.

These new make targets are used to create and run the valgrind tests as part of the normal test suite.