All user provided code is executed in an isolated container. ScriptRunner for Jira Cloud is a service that is shared by many customers so isolation is very important when executing scripts. Isolation is a goal that has been core to the design of ScriptRunner for Jira from its inception.
Each customer has a container that exclusively belongs to them. It is not possible for the code of one customer to have been run in the container of another. Containers are also designed to be single use, however, it is possible that containers will be re-used for execution on occasion, but script writers should not rely on this. Disk storage in these containers is short-lived, therefore, any changes written to the disk will be discarded.