Snapcraft PR: add official `core18` docker images

[sergiusens]

This is why:

Snapcraft docker missing set-version information

It is another test candidate added to our matrix. Snapcraft using base: core18 also relies a lot on build-snaps, even internally, so special tuning most likely be required in most cases and at that point, the generic solution is not that generic.

I will update the PR, the PR however depends on a previous PR (or at least the changes would cause conflicts), I only noticed now that it has been addressed. That said, this does not mean we will start publishing images.