Auto-trigger docker build for Linting tool for CloudFormation templates when new release is announced

The latest docker tag is the latest release verison (https://github.com/cfn-nag/cfn-nag/releases/latest)

Please avoid to use latest tag for any production deployment. Tag with right version is the proper way, such as alpine/cfn-nag:0.5.33

https://github.com/alpine-docker/cfn-nag

https://travis-ci.com/alpine-docker/cfn-nag

https://hub.docker.com/r/alpine/cfn-nag/tags/

# mount local folders in container.
docker run -ti --rm -v $(pwd):/apps -w /apps alpine/cfn-nag -i .

# run container as command
alias cfn_nag_scan="docker run -ti --rm -v $(pwd):/apps -w /apps alpine/cfn-nag"
cfn_nag_scan --help

--------
| WARN W47
|
| Resources: ["SNSAllConfigurationTopic", "SNSNotification"]
| Line Numbers: [45, 76]
|
| SNS Topic should specify KmsMasterKeyId property
------------------------------------------------------------
| FAIL F5
|
| Resources: ["AdditionalManagedIAMPolicy"]
| Line Numbers: [42]
|
| IAM managed policy should not allow * action
------------------------------------------------------------

https://stelligent.com/2018/03/23/validating-aws-cloudformation-templates-with-cfn_nag-and-mu/

Mostly it is used during CI/CD (continuous integration and continuous delivery) or as part of an automated build/deployment

• Enable Travis CI cronjob on this repo to run build daily on master branch
• Check if there are new tags/releases announced via Github REST API
• Match the exist docker image tags via Hub.docker.io REST API
• If not matched, build the image with release version and push to https://hub.docker.com/
• Get the latest version from https://github.com/stelligent/cfn_nag/releases/latest, pull the image with that version, tag as alpine/cfn-nag:latest and push to hub.docker.com