breaking: use a ltsc2019 base image for windowsservercore-ltsc2019 agent images and add a nanoserver-ltsc2019 image

[lemeurherve]

This PR refactors the build like what have been done in jenkinsci/docker-agent#459, with proper (new) windowsservercore-1809 and windowsservercore-ltsc2019 images (with corresponding base images).

It also add a nanoserver-ltsc2019 image, same as jenkinsci/docker-agent#488.

And it updates the docker-agent parent image version to the one including nanoserver-ltsc2019 & windowsservercore-1809 variants. (Supersedes and closes #398)
https://hub.docker.com/r/jenkins/agent/tags?page=1&name=nanoserver-ltsc2019

While I was at it and since these Windows images are just "patches" on top of the docker-agent ones, I've also regrouped both nanoserver and windowsservercore in a unique Dockerfile, simplifying a bit more the logic.
Unfortunately it's not possible to get env var substitution with the exec form of ENTRYPOINT like ENTRYPOINT ["${CONTAINER_SHELL}", "-f", "C:/ProgramData/Jenkins/jenkins-agent.ps1"]: https://docs.docker.com/engine/reference/builder/#exec-form-entrypoint-example
(Changing it to a shell form would be a breaking change too important).

_{FWIW, screenshot unique Dockerfile tentative}

Testing done

https://ci.jenkins.io/job/Packaging/job/docker-inbound-agent/job/PR-396/16/ ✅

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[lemeurherve]

Commenting out the test which fails here because of 0x1b escape character, crashing Pester when it tries to generate the XML export for JUnit: 9cd8ecb

Works correctly on my machine

Will be dealt with later.

[dduportal]

Works correctly on my machine

Good new, it's a container image

LGTM! Let's follow up on the pester "character" issue in jenkinsci/docker-ssh-agent#295 (comment)