Error docker.execute with str

[kashyab12]

#463

Getting rid of str annotations from command variables. I did notice that the execute method for the ContainerCLI class explicitly asserts for the command var to be of list type and throws an appropriate exception message if it is not. I decided not to perform the same assertion within the ComposeCLI class's execute function and the ServiceCLI class's create function since beartype can be of value of here (see comments within #466) . However, I am open to feedback regarding this.

[gabrieldemarmiesse]

Thank you for the pull request. Indeed, I'm still thinking about beartype from time to time, weighting the pros and cons (we might loose duck typing unless we use more complex types, like protocols).

Anyways, your PR is a strict improvement compared to the previous behaviour, so let's merge it