Use specific tag for jobs on k8s

On Kubernetes, if you specify a CI/CD job to use an image with a tag of latest, Kubernetes will default to imagePullPolicy: Always, but if you use a specific version, it will default to a pull strategy of IfNotPresent, which can save a lot of download time when nothing has changed. So while it's convenient to specify things like alpine, which implies alpine:latest, performance might be increased by using alpine:3.6. It's also a best practice for other reasons.

If this is significant, we should probably change our .gitlab-ci.yml templates, Auto DevOps template, and our documentation.

/cc @bikebilly