Grocker

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- Python 2.7 support (for Grocker command) will be dropped in next major version (**deprecation warning**)
- Accept wheel path as release argument

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- Add support for Python's stretch+3.6 docker image.
- Deprecate stretch/3.5 runtime

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- Use latest pip version (10.0)

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- Switch to Docker(-py) 3.0

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- Remove grocker version from the image default tag

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- Allow to set environment variables on the final image
- Deprecate Jessie runtimes
- Allow to use Debian Jessie (Python 2.7 et 3.4), Debian Stretch (Python 3.5) and Alpine (Python 3.6) as base distribution (**breaking change**)
- Simplify dependency declaration (**breaking change**)
- Manage HTTPError when using get or build image