User provided OPENJ9_JAVA_OPTIONS env var overrides OpenJ9 SCC options

https://github.com/AdoptOpenJDK/openjdk-docker/pull/338 added the ability to embed a shared-class-cache (SCC) in OpenJ9 containers. This OpenJ9 JVM feature is enabled by setting the following env var:
```
ENV OPENJ9_JAVA_OPTIONS="-Xshareclasses:name=openj9_system_scc,cacheDir=/opt/java/.scc,readonly,nonFatal"
```
However, if the user runs an OpenJ9 container image with `-e OPENJ9_JAVA_OPTIONS="..."` it may inadvertently remove the options that enable the embedded SCC.

I see two possible solutions:
1. Append the user provided options to the ones set in the container
2. Use a different env var for setting OpenJ9 options in container (for instance `JAVA_TOOL_OPTIONS` which already is used)

Attn: @bharathappali @dinogun