Migrate platform Docker image build to Kaniko

Currently using Docker with Skaffold, see https://github.com/nuxeo/nuxeo/pull/3183

Better to use Kaniko than the Docker from the K8s node:

• Control the Docker engine version and be able to use a recent one, allowing use of multi-stage, ...
• Don't monopolize the Docker resources: disk space for cache, etc.
• Leverage the Docker registry cache.
• Ability to build images in parallel.

We might also want to see if we can avoid calling Skaffold twice (this building the image twice) to push the image to the internal Jenkins X Docker registry and to the public one (packages.nuxeo.com): https://github.com/nuxeo/nuxeo/pull/3183/files#diff-58231b16fdee45a03a4ee3cf94a9f2c3R170

In fact currently we are pushing PR versions to the public registry whereas it could be in separate stage only for the master branch.