Always build latest Docker image during CI builds

[ChiefGokhlayehBosch]

I can't stand having to manually build docker images and fiddeling with credentials to push them to the hub.

Common practise is to build the Dockerfile for each job. Using tricks like
docker pull <some_image> && docker build ... --cache-from <some_image>, building can be sped up significantly, leading to a low penalty for building the image every time.

I propose the following naming scheme for Docker images on Docker Hub:

Name	Examples	Purpose
latest	-	Always reflects latest master. A rolling tag getting updated with every push/merge to master. The latest and greatest.
YY.MM	20.02, 20.08, 21.04	Stable release following every regular Kiso release (monthly, semi-annual, whatever)
stable	points to 21.04	Always points to the latest YY.MM tag.
build_X	build_281	Unique build number (equivalent to Jenkins build number). These may be used only temporary during a build. They are used as cache between build steps, if local caching is not feasible. Docker Hub might not be the fastest registry for this purpose and their use should therefore be limited/avoided. Their average expected lifetime is 5-15 minutes.

The procedure would run as the first build stage in the Jenkins Pipeline. Execution would occur on a Docker-out-of-Docker (dood) container on Eclipse's Kubernetes infrastructure. Build the image using ... --cache-from kiso-build-env:latest for performance, instantiate a container with it and if build is happening on master, tag it latest and push it to the hub.

Subsequent steps shall then be executed inside the freshly instantiated container.

[ChiefGokhlayehBosch]

I'm by far no expert on Kubernetes CI infrastructures but I see that even when I change the pod-description in the Jenkinsfile, the Eclipse CI still instantiates a container of the old image tag v0.0.1. This means even if I update the Dockerfile, build a new tag and change the reference in the Jenkinsfile, Eclipse CI will still run with an old Docker image, potentially tainting the build.

@elbosch @HansUweRempler, just want to bring this to your attention. If we are not allowed to fiddle with the infrastructure ourselves, we have to contact Eclipse about that.