docker: disable driver when running as non-root on cgroups v2 hosts

[shoenig]

This PR modifies the docker driver to behave like exec when being run
as a non-root user on a host machine with cgroups v2 enabled. Because
of how cpu resources are managed by the Nomad client, the nomad agent
must be run as root to manage docker-created cgroups.

Fixes #15927

[shoenig]

Spot check

running as non root

➜ ps -ef | grep -v grep | grep nomad | fields 1
shoenig

Drivers
Driver       Detected  Healthy  Message                  Time
docker       false     false    Driver must run as root  2023-02-06T11:21:06-06:00
exec         false     false    Driver must run as root  2023-02-06T11:21:06-06:00
java         false     false    Driver must run as root  2023-02-06T11:21:06-06:00
mock_driver  true      true     Healthy                  2023-02-06T11:21:06-06:00
qemu         false     false    <none>                   2023-02-06T11:21:06-06:00
raw_exec     true      true     Healthy                  2023-02-06T11:21:06-06:00

running as root

Drivers
Driver       Detected  Healthy  Message  Time
docker       true      true     Healthy  2023-02-06T11:22:29-06:00
exec         true      true     Healthy  2023-02-06T11:22:29-06:00
java         true      true     Healthy  2023-02-06T11:22:29-06:00
mock_driver  true      true     Healthy  2023-02-06T11:22:29-06:00
qemu         false     false    <none>   2023-02-06T11:22:29-06:00
raw_exec     true      true     Healthy  2023-02-06T11:22:29-06:00