make command optional on image creation

The _command_ parameter of `CreateContainerCommand` is always applied to the container, whether it is set or not.

This causes two problems when subsequently starting the container:
* if you leave the _command_ parameter empty, startup fails with 
_Cannot start container [...]: exec: "": executable file not found in $PATH_
* if you specify a _command_, the container executes it instead of the one from the images's `CMD` setting.

Therefore you cannot use preconfigured startup commands in images.

With this PR, the _command_ parameter is only applied when it is specified.