Accounts for the 2.9.10 security release and allows testing Traefik 3 betas.
* Use `image` to configure a specific Traefik Docker image.
* Default to `traefik:v2.9` to track future 2.9.x minor releases rather
than tightly pinning to `v2.9.9`.
* Support images from the configured registry.
References #165
Commander had version/destination solely to incrementally accumulate CLI
options. Simpler to configure in one shot.
Clarifies responsibility and lets us introduce things like
`abbreviated_version` in one spot - Configuration.
Less work for broadcast commands to take on.
Also fixes a bug where rollback on hosts without a running container
would stop the container they had just started.
Jeremy Daer