[PATCH 3/3] docker: Use pyenv for Python versions

[Stephen Finucane]

On Tue, 2019-09-03 at 18:03 +0100, Stephen Finucane wrote:
> This is slightly slower to initially configure but requires less hacking
> to get the same environment and should be a lot more maintainable (just
> a simple modification to change the Python version).
> 
> Signed-off-by: Stephen Finucane <stephen at that.guru>

This is somewhat more complex than the other two patches in the series,
but I've been playing around with it locally and it seems to do what I
wanted. I've gone ahead and applied this on the assumption that we can
easily revert it if necessary.

Stephen