FEATURE: add runtime features (#849)

FEATURE: add runtime features (#849)

adds commands for: start, run, stop, cleanup, destroy, logs, enter, restart, rebuild -- carrying over existing run commands from launcher1.

Rebuild will also do its best to minimize downtime with the following steps:

* Detect if Discourse is running as a single container or external DB
* Detect if db:migrate is configured to run on container boot
* Build initial container (keeping existing one online)
* Exit running containers if it's a single container (otherwise keeps existing online)
* Run migrations
  * Defer migrations if db:migrate is configured to run on container boot
  * Run migrations with SKIP_POST_DEPLOYMENT_MIGRATIONS=1 if it's a 2 container setup
  * Otherwise, run all migrations
* Destroy the old container (finally stopping the current, if it's still up here)
* Start the new container
* Run post-deploy migrations
  * Run migrations with SKIP_POST_DEPLOYMENT_MIGRATIONS=0 if it's a 2 container setup