Update to latest crossbar base Docker image

[BrianJKoopman]

Description

This PR catches up to the latest release from crossbar on Docker Hub. There have been some changes in crossbar and how they're releasing it in the couple years since we last updated so this also:

• sets auto_ping_size to the new minimum
• does some user creation within the image that used to be done by crossbar, but is missing in the new images

Motivation and Context

We've been running version 20.8.1 of crossbar for several years now without updating.

#290 was attempting to auto-update this (and continue to do so moving forward), however it's hitting the latest in that sort of version structure crossbar used to use, so version being auto PR'd is 2 years old even.

How Has This Been Tested?

I've run this new image in a small 5 agent system on my development box.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• Unless I am preparing a release, I have opened this PR onto the develop branch.