Update to Ubuntu 20.04 (Focal) LTS

[keeganwitt]

(closes #354)

[karianna]

LGTM but we can only land this post 15 release

[keeganwitt]

If we were to publish both 18.04 and 20.04 based images, would we be able to do prior to 15?

I'm assuming by your comment that we would publish 20.04 images for 15 but not for 8 or 11? I use this as a base image for the official Gradle image, and I'm guessing most folks would want to stick with an LTS Java, but want the new base image for things like Git (it is currently not usable for certain things, like GitHub actions) because the Git in the Ubuntu repo is too old.
Otherwise I'm gonna have to install Git from a PPA or source, which is not ideal.

If I misunderstood and after 15 we plan to release images for 8 and 11 based on 20.04, then I am less interested in publishing both variants.

[aahlenst]

You misread Martijn. We don't merge anything (except emergency fixes) during release weeks (which takes extra long this time because OpenJ9 isn't ready). So "post 15" means you have to wait until release week is over.

[keeganwitt]

Rebased.

For those following the issue, here's the MR for 15: docker-library/official-images#8781