Added env variable to install Splunk ESCU v4.25.0

[p1nkus]

Wanted to try to include some additional functionality in the base installation of the lab. After trial and error with a few different ideas to load in detection rules, ended up finding this documentation which led to the proposed adjustment:

https://splunk.github.io/docker-splunk/advanced/APP_INSTALL.html#download-via-url

Please let me know if there are any other features you had in mind for the project that you think would be valuable!