Add travis-ci docker image pipeline

[ghost]

Resolves https://github.com/mcguinness/saml-idp/issues/53

Unable to fully test this as TravisCI does not run on forks, and Docker Hub credentials need to be setup.

[ghost]

@JaneJeon I can look into some options but it does look like there have been several updates this year to the repo by @mcguinness and others. I would prefer to have the image be based upon the latest source if possible and updated automatically via CI/CD (Maybe GitHub Actions now instead of Travis CI).

If you need an image before then I would recommend building and pushing up your own. This has the added benefit of using a stable commit you know works for you.

https://github.com/mcguinness/saml-idp#docker

# assuming you are in root of repo
docker build . -t janejeon/saml-idp:latest
docker push janejeon/saml-idp:latest

[mcguinness]

I am maintaining the code. Is the ask here just to publish an image to docker hub on each release? What else is needed besides a .travis.yml file for CI/CD workflows? I am assuming the Dockerfile in the repo is fine, does that need any changes?

[ghost]

@mcguinness I think everything Docker related is already good to go, just the CI/CD workflow to publish an image on each release.