[Roadmap] Support multiple engine types

[olljanat]

For some unknown reason Rancher has used both terms “engine” and “user docker” when referring the Docker daemon which are used to running containers.

In readme we have this picture:

But same time command which is used to manage Docker version is actually ros engine and in index.yml file located on os-services all those have docker- prefix.

RancherOS also had support for running multiple user dockers in parallel. That still exist in our documentation and code but most likely will not work.

So right thing to do in next major release (if that happens) is that code would be made more flexible for additional engine types and none of those should not be enabled by default.

That would mean that K3s support can be handled by adding new engine type and preferably it should be possible by updating just code in os-services. Similarly it should be possible to run virtual machines on top of BurmillaOS by adding engine type QEMU.

[olljanat]

In practice it is this part in main config of OS which defines which engine is used:

os/os-config.tpl.yml

Lines 372 to 396 in 29a661b

	docker:
	{{if eq "amd64" .ARCH -}}
	image: {{.OS_REPO}}/os-docker:{{.USER_DOCKER_VERSION}}{{.SUFFIX}}
	{{else -}}
	image: {{.OS_REPO}}/os-docker:{{.USER_DOCKER_VERSION}}{{.SUFFIX}}
	{{end -}}
	command: ros user-docker
	environment:
	- HTTP_PROXY
	- HTTPS_PROXY
	- NO_PROXY
	labels:
	io.rancher.os.scope: system
	io.rancher.os.after: console
	net: host
	pid: host
	ipc: host
	uts: host
	privileged: true
	restart: always
	volumes_from:
	- all-volumes
	volumes:
	- /sys:/host/sys
	- /var/lib/system-docker:/var/lib/system-docker:shared

and it is overwritten by docker version specific files in os-services repo like this

So main change needed afaiu is that instead of using ros user-docker command to launch those engines, we should create separate Git repo to provide ros-engine binary so that one can be then included to each of those engine container images and new/updated engine types can be then introduced without need for new OS version.

It will also make simpler to handle cases like #161 (comment) where new Docker version changes requirements.

[olljanat]

First prototype based on ea0c559 seems to be working.

User Docker not there and it is running KubeSolo instead of (manually started to console):

Biggest issue seems to be that ros engine switch command fails as it expect that docker service would be defined in OS main config.

EDIT: This prototype exists in develop branch.