Fix a few bugs in RMI server

[StroppaFR]

This fixes a few bugs, some preventing the tool from working correctly:

• Initialize customScript in RMIRefServer to avoid NullPointerException when using the RMI server.

2026-07-21 20:52:13 [RMISERVER]  >> Sending local classloading reference for BypassByEL.
java.lang.NullPointerException
        at jndi.RMIRefServer.execByJavaxELNashomTomcatOld(RMIRefServer.java:397)
        at jndi.RMIRefServer.handleRMI(RMIRefServer.java:328)
        at jndi.RMIRefServer.doCall(RMIRefServer.java:282)
        at jndi.RMIRefServer.doMessage(RMIRefServer.java:234)
        at jndi.RMIRefServer.run(RMIRefServer.java:186)
        at java.lang.Thread.run(Thread.java:750)
2026-07-21 20:52:13 [RMISERVER]  >> Closing connection

• Update base Docker image to eclipse-temurin:8-jre-alpine because openjdk:8-jdk-alpine does not exist anymore.
• Fix displayed RMI server address.