Make default Dockerfile embedded

[ssbarnea]

From now on, creating a Dockerfile for docker/podman containers would
be optional. If the file is not present molecule would use its internal
template.

This ease maintenance of roles using molecule by having one less file
to create and update.

Those relying on custom containers are not affected, as this feature is
still present.

Newly created roles will not have Dockerfiles.

[decentral1se]

Haven't tested it but this looks good and is a useful change!

If you can move the vars into the ansible.py that would be better though, if it makes sense.

[decentral1se]

What about feeding it in from:

molecule/molecule/provisioner/ansible.py

Lines 593 to 601 in 97a7b60

	molecule_vars = {
	'molecule_file': "{{ lookup('env', 'MOLECULE_FILE') }}",
	'molecule_ephemeral_directory': "{{ lookup('env', 'MOLECULE_EPHEMERAL_DIRECTORY') }}",
	'molecule_scenario_directory': "{{ lookup('env', 'MOLECULE_SCENARIO_DIRECTORY') }}",
	'molecule_yml': "{{ lookup('file', molecule_file) | molecule_from_yaml }}",
	'molecule_instance_config': "{{ lookup('env', 'MOLECULE_INSTANCE_CONFIG') }}",
	'molecule_no_log': "{{ lookup('env', 'MOLECULE_NO_LOG') or not "
	"molecule_yml.provisioner.log|default(False) | bool }}",
	}

Does it work?