Backport of #46599 and #47640. Add packaging tests for Docker.
* Introduce packaging tests for Docker (#46599)
Closes#37617. Add packaging tests for our Docker images, similar to what
we have for RPMs or Debian packages. This works by running a container and
probing it e.g. via `docker exec`. Test can also be run in Vagrant, by
exporting the Docker images to disk and loading them again in VMs. Docker
is installed via `Vagrantfile` in a selection of boxes.
* Only define Docker pkg tests if Docker is available (#47640)
Closes#47639, and unmutes tests that were muted in b958467.
The Docker packaging tests were being defined irrespective of whether
Docker was actually available in the current environment. Instead,
implement exclude lists so that in environments where Docker is not
available, no Docker packaging tests are defined. For CI hosts, the build
checks `.ci/dockerOnLinuxExclusions`. The Vagrant VMs can defined the
extension property `shouldTestDocker` property to opt-in to packaging
tests.
As part of this, define a seperate utility class for checking Docker,
and call that instead of defining checks in-line in BuildPlugin.groovy
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