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
1f3d1019e7