Teach the run.sh to use START_CMD if set, even when SNAPSHOT_PATH is unused
(previously, it would exec whatever command line passed to the docker, or do
nothing if none).

This also fixes the case of COSMOVISOR_ENABLED=1, which sets START_CMD to
start cosmovisor but then was not actually run before.

Users generally want to:

1. Run the blockchain binary either as-is, -or- with snapshot.sh, -or- with
cosmovisor. These are controlled `SNAPSHOT_PATH`, and `COSMOVISOR_ENABLED`.

and then:

2. Be able to set the blockchain binary command to something else than the
default "start". This can be done with `START_CMD` - to amend/override that
command via environment variables without changes to deployment logic - e.g.
docker-compose yml (unlike with command line args to "docker run ...").

So change the logic to be: "use rule 1 above for prefix, then if args exists
then use them, else if `START_CMD` exists then use it, else use `start`". I
amended the PR accordingly.

With the "CMD $START_CMD" directive ("shell form") docker starts the container
with "$ENTRYPOINT /bin/sh -c $START_CMD". This is bad because it breaks both
cosmovisor and snapshot.sh (the latter has an explicit workaround). It is also
unnecessary because the $START_CMD can (and should) be enforced by `run.sh`.

Remove "CMD $START_CMD", since it is passed in the env and handled anyhow by
`run.sh`. Replace with "CMD []" to un-inherit any "CMD" directives from parent
images.