As many others before us we have problems with @ in workspace paths for concurrent jobs. Setting the property hudson.slaves.WorkspaceList in JAVA_OPTS works but is a bit of a hassle and requires some extra tweaking in our docker setup script.
A better way for us is to use System.setProperty() to set it in the startup script but that has no effect. System.getProperty() verifies the new value but the default value is still used. I consider this a bug.
An even better way would be to promote this to a normal setting. The system property documentation mentions
"In general, these switches are often experimental in nature, and subject to change without notice. If you find some of those useful, please file a ticket to promote it to the official feature."
and a lot of people seem to be interested in changing this property so it should be promoted to an official setting. This would then be a feature request.