Add Instance Number to Environment Variables

[rakstar]

I run multiple builds on a single host by setting "Instance Capacity" > 1. While this works fine, I have a monolithic app that takes a while to do a fresh git clone with each build. I could speed this up by bind mounting my "Remote File System Root" to a volume so that previous git clones can be reused but, as far as I can tell, this currently wouldn't work properly with multiple concurrent instances.

I think I could address this by having a volume mount declaration like:
type=volume,src=jenkins_agent,dst=/home/jenkins/agent_$INSTANCE_NUMBER

OR perhaps by having a static volume mount and setting my Remote File System Root to something like:
/home/jenkins/agent/workspace_$INSTANCE_NUMBER

But for the above to work, I believe the return value of the following method needs to be saved to the INSTANCE_NUMBER environment variable:

docker-plugin/src/main/java/com/nirima/jenkins/plugins/docker/DockerCloud.java

Line 341 in ae073a3

int countContainersInProgress() {

[FROSADO]

I have a similar request : #929