Docker run command needs path to image

The docker image requires the path used during pull to be provided as part
of the image name else the following error is received:

Unable to find image 'apertis-v2021dev1-package-source-builder:latest' locally
docker: Error response from daemon: pull access denied for apertis-v2021dev1-package-source-builder, repository does not exist or may require 'docker login': denied: requested access to the resource is denied.
See 'docker run --help'.

Update the documentation to use the correct command.

Signed-off-by: Martyn Welch <martyn.welch@collabora.com>