Add Concourse CI/CD Pipelines

[nderjung]

This PR includes tools, scripts and templates for CI/CD pipelines for kraft based on Concourse which is hosted for the Unikraft OSS project at https://concourse.unikraft.org. The pipelines themselves serve for packaging, testing and distributing kraft. Currently this is limited to a suite of Docker images and a few .debs for LTS Ubuntu/Debian distributions.

Pipelines

There are 3 pipelines, 2 are based on the two primary branches of the kraft repository (stable and staging) and the last pipeline is designed for all PRs made here on GitHub. Of the three pipelines, all generate similar artifacts (iterated in detail below). For now, there are two categories: Docker images and Debian/Ubuntu distributables.

For Docker images, a fully packaged environment is prepared under unikraft/kraft available from docker.io. For Debian images, a seperate APT repository is hosted at http://releases.unikraft.org which can be used like:

sudo add-apt-repository \
   "deb [arch=amd64] http://releases.unikraft.org/linux/$DISTRO \
   $(lsb_release -cs) \
   $PIPELINE"

Where $DISTRO is either ubuntu or debian and $PIPELINE is one of the three listed below:

stable branch

This first pipeline is designed to release, as per the branch name, stable versions of kraft dictated by a semver bump. It prepares -smoke candidates of all artifacts which are only released when the relevant test has passed.

Distributables

Vendor	Distributable	Purpose
Docker	docker.io/unikraft/kraft:latest	The latest stable version of the kraft tool.
	docker.io/unikraft/kraft:$APP_VERSION
	docker.io/unikraft/kraft:latest-dev	The developer release of the kraft Docker environment, with extra tools for the maintenance of kraft itself.
	docker.io/unikraft/kraft:$APP_VERSION-dev
Ubuntu	unikraft-tools_$APP_VERSION~xenial_all.deb	Xenial (16.04) LTS
	unikraft-tools_$APP_VERSION~bionic_all.deb	Bionic (18.04) LTS
	unikraft-tools_$APP_VERSION~focal_all.deb	Focal (20.04) LTS
Debian	unikraft-tools_$APP_VERSION~stretch_all.deb	Stretch 9
	unikraft-tools_$APP_VERSION~buster_all.deb	Buster 10 (stable)

staging branch

This next pipeline is targeted towards the latest commits which have just been merged from PRs into the kraft source tree. These may not necessarily be stable features and so merging into this tree from a PR may still result in errors in the pipeline but is done so to guarantee faster feature access via one of the distrbutables.

Distributables

Vendor	Distributable	Purpose
Docker	docker.io/unikraft/kraft:staging	The latest staging version of the kraft tool.
	docker.io/unikraft/kraft:staging-dev	The developer release of the kraft Docker environment, with extra tools for the maintenance of kraft itself.
Ubuntu	Same as above but distributed to different APT repositories.
Debian

testing Pull Requests (Including this one!)

Vendor	Distributable	Purpose
Docker	docker.io/unikraft/kraft:pr-$PR_ID	The latest staging version of the kraft tool.
	docker.io/unikraft/kraft:dev-pr-$PR_ID	The developer release of the kraft Docker environment, with extra tools for the maintenance of kraft itself.
Ubuntu	Same as above but distributed to different APT repositories.
Debian