Enable ccache in UDF examples build

[GaryShen2008]

Install ccache in Dockerfile used to build UDF example.
Update README to explain how to use it by adding some environment variables

close #343

[GaryShen2008]

One problem may be if we need to directly explain how to enable a persistent cache folder by mounting one folder to the docker like https://github.com/NVIDIA/spark-rapids-jni/blob/branch-24.02/build/run-in-docker#L66-L70.
I just give a simple sample to enable ccache and suggest to use a mounted folder, but I don't give the full commands.
I'd like to have the review first, if it's better to use a mounted folder, I can update this PR.

[jlowe]

A mounted folder would be nice, but this significantly increases the complexity of running the container unless there's a run script like we have for spark-rapids-jni. I'm OK with leaving this out for the sake of simplicity, but we might want to point to the spark-rapids-jni example for how to do this.