Load runtime data only on invocation

[JosepSampe]

Kubernetes backends (kubernetes, code_engine and knative) needs docker installed in the local machine to build the default runtime if no runtime is provided in config. In this sense, all these backends (config.py files) contains a block of logic that checks whether docker is installed or not. This is a limitation when using the lithops cli, since for almost all the lithops cli commands it is not needed to check whether docker is installed or not.

This patch moves the logic that checks if docker is installed within a method that it only called during the invocation.

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