[Documentation] Clarify the use of Docker within shared and private VMs
The Port a Pipeline wiki page describes the process of installing Docker to build supporting images. This only works in cases where the user has a shell VM dedicated to them—which is not always the case, and not the usual case on our public beta.
Update the documentation on the assumption that the site sysadmin is in full control over Docker installation and configuration on the box:
- Remove the section that describes how to install Docker.
- In the portions of the documentation about preparing a Docker image, note that access to Docker is a prerequisite, and may not be available on Arvados shell nodes depending on site security policy. If they don't have access to Docker through a shell node, the steps can also be done on a local workstation with Docker and the Python SDK installed (link to the corresponding documentation for installing both of these). Users can build a Docker image locally and upload it to Arvados when they're done with