Merge memory related Frank!Framework image JAVA_OPTS with our own memory optimalizations in entrypoint

[MLenterman]

We recently moved back to using the Frank!Framework Docker image as base image, instead of building our own Docker image from scratch. This was done to have more freedom with memory optimization and Java versions. The Frank!Framework base image uses a different garbage collection with different JAVA_OPTS now and run on Java 17. Currently we are still overriding the Frank!Framework image JAVA_OPTS with our own. The goal is to primarily use the JAVA_OPTS from the Frank!Framework image and add our ZaakBrug specific memory optimizations on top of them.

• Merge JAVA_OPTS
• Validate with performance tests

[MLenterman]

Blocked by #237