Make the hadolint task in travis.yml less fixed, enable it to run recursively.

Is your feature request related to a problem? Please describe. I'm annoyed by the way I've locked down the hadolinting task in the travis.yml file, currently it is being run 3 times with the Dockerfile-*'s being piped in individually, this has a number of issues, the main one being that if the file is renamed or new Dockerfile's are added, linting will no longer occur on the files. Meaning some items will not get linted.

Describe the solution you'd like Enable hadolint to run across the entire directory, picking up what it sees to be a valid Dockerfile and linting it in an appropriate manner.

Describe alternatives you've considered Build our own hadolint service in docker-services that can ingest directories as targets and include it within our travis file.

Additional context Overall, nothing else. I'd give this 2 points, seeing as it's not an instant fix, and require something. I'd give the task 3 points if a separate docker service had to be written.