Always use semantic versions for referencing third-party images

[petermikitsh]

Within snow's codebase, we reference the latest tag for the kaniko image [0].

e.g:

            "containers": [
              {
                "name": "kaniko",
                "image": "gcr.io/kaniko-project/executor:latest", <--- let's not do this
                "args": [
                  "--context=dir:///kaniko/build-context",
                  "--destination=${imageName}",
                  "--skip-tls-verify"
                ],

This poses a problem if Kaniko ever introduces any breaking changes. We should specify a particular version, and upgrade it as necessary / cut new releases of Snow so users can also begin using newer versions of Kaniko.