docker: exclude 'entrypoint' from comparing 'command' param with containers

The JSON the Docker API returns includes the container's ENTRYPOINT value (if it has one) with the 'Command' value. So instead of checking if `container['Command'] == module.params['command']`, we just check that `container['Command'].endswith(module.params['command'])` so the entrypoint won't affect a container being properly classified as matching the module params or not.

Also I refactored a super-long `if` statement into some temporary variables - I did it to help me figure out what was going wrong, and then it makes the code more readable so I kept it.
Merge pull request #7047 from joshuaconner/docker_bugfix_exclude_entrypoint_from_command_check

docker: exclude 'entrypoint' from comparing 'command' param with containers