Fix issues with the shell style CMD format, eg `CMD echo test`

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I've tested this with CMD echo test, CMD ["echo", "test"], and CMD ["echofail", "test"] and they all work as expected, the only one I'm unsure of is CMD echofail test gives me /bin/sh: 1: echofail: not found - however on the latest rpi image it gives test: 1: test: echofail: not found so it seems ok

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Pushed another fix such that CMD ["echo test"] now gives Command not found: echo test

[petrosagg]

SGTM