### What changes were proposed in this pull request?
This PR aims to update the docker/spark-test and clean up unused stuff.
### Why are the changes needed?
Since Spark 3.0.0, Java 11 is supported. We had better use the latest Java and OS.
### Does this PR introduce _any_ user-facing change?
No.
### How was this patch tested?
Manually do the following as described in https://github.com/apache/spark/blob/master/external/docker/spark-test/README.md .
```
docker run -v $SPARK_HOME:/opt/spark spark-test-master
docker run -v $SPARK_HOME:/opt/spark spark-test-worker spark://<master_ip>:7077
```
Closes#29150 from williamhyun/docker.
Authored-by: William Hyun <williamhyun3@gmail.com>
Signed-off-by: Dongjoon Hyun <dongjoon@apache.org>
## What changes were proposed in this pull request?
Move `docker` dirs out of top level into `external/`; move `extras/*` into `external/`
## How was this patch tested?
This is tested with Jenkins tests.
Author: Sean Owen <sowen@cloudera.com>
Closes#11523 from srowen/SPARK-13595.
