use tini to run every container (#1195)

fixes #1101

additional context:

* this was introduced in docker 1.13 (1/2017), we require docker 17.10
(10/2017), this should not have any issues dependency-wise, as `docker-init`
is in the docker install from that point in time. unless explicitly removed,
it should be in the dind container we use as well...
* the PR that introduced this to docker is
https://github.com/moby/moby/pull/26061 for additional context
* it may be wise to put this through some paces, if anybody has any...
interesting... function containers. the tests seem to work fine, however, and
this shouldn't be something users have to think about (?) at all, just
something that we are doing. this isn't the default in docker for
compatibility reasons, which is maybe a yellow flag but I am not sure tbh

api/agent/drivers/docker/docker.go

@@ -223,6 +223,7 @@ func (drv *DockerDriver) CreateCookie(ctx context.Context, task drivers.Containe
 		},
 		HostConfig: &docker.HostConfig{
 			ReadonlyRootfs: drv.conf.EnableReadOnlyRootFs,
+			Init:           drv.conf.EnableTini,
 		},
 		Context: ctx,
 	}