Rewrite docker to be sensible and smaller

I have checked the image can build things and inspected `diff -ru`
compared to the old image. As far as I can tell it is more or less
the same besides the later git change.

Layers are now 65MB or less, and we aren't against the maxLayers limit
for the broken automatic layering to do anything but shove one store
path in a layer (which is good behaviour, actually).

This uses nix2container which streams images, so the build time is much
shorter.

I have also taken the opportunity to, in addition to fixing the 400MB
single layer (terrible, and what motivated this in the first place),
delete about 200MB of closure size inflicted by git vs gitMinimal
causing both perl and python to get into closure.

People mostly use this thing for CI, so I don't really think you need
advanced git operations, and large git can be added at the user side if
really motivated.

With love for whichever container developer somewhat ironically assumed
that one would not run skopeo in a minimal container that doesn't have a
/var/tmp.

Fixes: https://git.lix.systems/lix-project/lix/issues/378

Change-Id: Icc3aa20e64446276716fbbb87535fd5b50628010

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