build: update docker image on every master commit that passes tests

[mattklein123]

[rshriram]

@mattklein123 this is easy IIRC. In dockerhub, there is a setup option that automatically builds the docker image anytime the master branch gets updated. https://docs.docker.com/docker-hub/github/ .

[mattklein123]

Need to also build all of the example images.

[mattklein123]

@junr03 per discussion in #416 as part of this work can we also get a script together for pushing the envoy-build image to dockerhub with a sha tag, then update do_ci.sh to explicitly use a tagged version of the build image.

[junr03]

@mattklein123 do we want to push the envoy-build image automatically (on a trvis build), or just have a script that we run locally when we update dependencies?

[mattklein123]

I think it can be manual, but we should push the image tagged at the SHA it was built at, and then reference that tag in do_ci.sh. Then we should remove the latest image so that all builds are explicitly version locked.

[junr03]

@mattklein123 ok sounds good. In that case do we also want to version lyft/envoy, or keep the understanding that we push on every push to master that passes tests?

[WhileLoop]

I would like to see tagged releases if possible.

[junr03]

@WhileLoop just to be clear, are you talking about versioned releases like lyft/envoy:1.0.0, or tagged releases when master passes tests like lyft/envoy:SHA?