[Snek] Updated github action -- includes container creation

[Keith Packard]

I've modified the snek github action script to construct a suitable
container right in the script rather than pulling one from docker. This
means the description of the container is now part of the snek source
repository and so any changes needed can be made with whatever caused
those changes to be needed.

As a result, the CI system is now running again (hurray!).

One related consequence is that I'm no longer able to test snek under
ARM qemu (alas!). That's because the current Debian unstable QEMU
(5.0-4) cannot run under docker due to a change in how it manages
memory. I expect that will get fixed, at which point we can update the
github action and docker container to include qemu and run tests under
it.

At least it's running tests on x86, which should catch at least some
things.

-- 
-keith
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