Raise an error to the user if the command parsed is blank

[mike-weiner]

Related to: #1399

weiner % kamal app exec --primary --env=TEST:1 '/usr/bin/echo :test'      
Get most recent version available as an image...
Launching command with version latest from new container...
  INFO [ab8c5ab1] Running docker run --rm --network kamal --env PORT="3000" --env TEST="1" --env /usr/bin/echo ="test" --log-opt max-size="10m" registry.hub.docker.com/<REDACTED>/<REDACTED>:latest  on <REDACTED>
  ERROR (SSHKit::Command::Failed): Exception while executing on host <REDACTED>: docker exit status: 125
docker stdout: Nothing written
docker stderr: docker: invalid reference format.
See 'docker run --help'.

option :env, aliases: "-e", type: :hash, desc: "Set environment variables for the command"

The problem described in #1399 appears to be a side effect of the env option being of type hash.

If the user's command to execute contains : and it appears after the first --env option in the command string, thor appears to try and (incorrectly) treat it as a key/value pair.

There should be a more informative error message returned to the user in this scenario. This problem is avoided altogether if the command to execute inside the container(s) is provided first followed by any options.