Provide a maven-java11-nuxeo Jenkins X builder

To allow the Nuxeo Platform build, aka the nuxeo GitHub repository.

We need at least:

• A Dockerfile describing the Maven container Docker image, based on the default one gcr.io/jenkinsxio/builder-maven-java11 and including the conversion tools.
• A Jenkinsfile for the pipeline building the builder.

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See https://github.com/nuxeo/nuxeo-jenkins-x-builders

In particular:

• the v0.0.1 release (v0.0.1 is the GitHub tag, the image tag is 0.0.1)
• the Dockerfile
• the Jenkinsfile
• the Skaffold build directive

Motivation to have a mono-repository for several builders (at the moment: "builder-maven-java11-nuxeo" and "builder-nodejs-njx"):

• Simpler to maintain (shared Jenkinsfile, scripts, skaffold.yaml, ...).
• Simpler to release: one release and same versioning for all the builders.
  Though releasing all builders for just a bug fix in one of them seems overkill, since most of the upgrades will be implied by a "jenkinsxio/builder-base" image upgrade, it makes sense to upgrade all the builders at one go.
• Ensures that all the builders are built on the same Jenkins X instance, thus with the same versions of the build tools such as jx, skaffold, etc.

Next steps:

• Use a "jenkins" user in the agent to not run the pipelines as root, see NXBT-2892.

• Currently the builders are built in the "platform" team's Jenkins X instance.
  We should build the builders, if generic enough, on a dedicated Jenkins X instance so that:
  • The version of the Jenkins X platform (and related tools, such as jx, skaffold, ...) is managed independently from a given team (fixed, then upgraded when wanted).
    In fact, the base Jenkins X image version is hardcoded and shared between all builders, see for instance the builder-maven-java11-nuxeo Dockerfile. We should have an automatic way to upgrade all our builders depending on the deployed Jenkins X platform version.
  • The builders don't get built by each team's Jenkins X instance.