Allow setting of Docker image tag when deploying without building

[petromir]

Hi, thanks for the great tool!

I'm trying to deploy an app without building the Docker image (I already have it), by running:

kamal deploy --skip-push

Unfortunately, I'm unable to set the Docker image tag (a custom one, e.g. 1.2.3) as Kamal always takes the full commit SHA

[petromir]

kamal deploy --skip-push --version=1.2.3

solved the issue