Add support for provider-end entrypoints

[shadeofblue]

It seems that, starting with version 0.12 (ya-runtime-vm 0.3), yagna providers will be able to automatically execute entrypoints in the images...

• [x] depends on https://github.com/golemfactory/ya-runtime-vm/issues/146
• [ ] add the capability in yapapi
• [ ] add support for the parameter in the Start command
• [ ] provide (modify?) an example in yapapi

Usage example:

• I am starting a Django application. I would like to create an entrypoint script, that will always execute migrations before executing the main CMD command (which most often would be the same)
• I want to add a supervisor that will execute a command provided in a CMD, but will store it's logs (stderr, stdout, exit code etc.) into a file (e.g. in a VOLUME path so that it can be later sent to requestor)

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Questions:

• [ ] What are "entrypoints in the images"