The `alpha watch` command current "attaches" to an already-running
Compose project, so it's necessary to run something like
`docker compose up --wait` first.
Now, we'll do the equivalent of an `up --build` before starting the
watch, so that we know the project is up-to-date and running.
Additionally, unlike an interactive `up`, the services are not stopped
when `watch` exits (e.g. via `Ctrl-C`). This prevents the need to start
from scratch each time the command is run - if some services are already
running and up-to-date, they can be used as-is. A `down` can always be
used to destroy everything, and we can consider introducing a flag like
`--down-on-exit` to `watch` or changing the default.
Signed-off-by: Milas Bowman <milas.bowman@docker.com>