Grocker

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- optionnal manifest set image hash to None

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- Drop Python 2.7 support (for Grocker command) **breaking change**
- Retrieving the image hash from manifest is now optional and disabled by default

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- Add Python 3.8 support (for Grocker command).
- Add support for buster/3.8 runtime.
- Add support for buster/3.7 runtime.
- Add support for alpine/3 runtime and use it by default.
- Deprecate stretch/3.7 runtime (**deprecation warning**).
- Deprecate alpine/3.6 runtime (**deprecation warning**)
- Drop Python 3.4 support since it is not supported anymore (**breaking change**).

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- Fix pip install error when building an image from a wheel with extra requirements passed from a filepath

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- Python 3.4 & 3.5 support (for Grocker command) will be dropped in next major version (**deprecation warning**)
- Add support for Python's stretch/3.7 docker image.

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- Replace "+" signs in python versions by "-" in default image name