Support configuring shared memory size via shm_size

[zleinweber]

The docker_container_module allows configuring a container's shared memory size via the shm_size parameter. However, as far as I can tell the molecule configuration for the docker driver does not support passing this through.

The default size that docker uses when this parameter is unset is 64M. I have a scenario where configuring this to a larger value would be useful for me. This is a feature request to support passing shm_size through to the backend docker_container_module via molecule platform configuration.