The 'docker.config.infra_image' would default to an amd64 container.
It is possible to reference the correct image for a platform using
the `runtime.GOARCH` variable, eliminating the need to explicitly set
the `infra_image` on non-amd64 platforms.
Also upgrade to Google's pause container version 3.1 from 3.0, which
includes some enhancements around process management.
Fixes#8926
* docker: support group allocated ports
* docker: add new ports driver config to specify which group ports are mapped
* docker: update port mapping docs
> If you do not run Nomad as root, make sure you add the Nomad user to the Docker group so Nomad can communicate with the Docker daemon.
Changing the username in the example from `vagrant` to `nomad`. Vagrant isn't addressed in the entire document, so I guess that this was a mistake.
