[Bug] apachepulsar/pulsar-build docker image is not maintained build/docker directory is obsolete

### Search before asking

- [X] I searched in the [issues](https://github.com/apache/pulsar/issues) and found nothing similar.

### Version

In the GitHub repository for branch-3.0, and master the directory is `build/docker` is obsolete.

### Minimal reproduce step

I looked for uses of the Dockerfile and https://hub.docker.com/r/apachepulsar/pulsar-build - the image is 9 months old.

### What did you expect to see?

Either the README states this is obsolete, or the directory is removed.

### What did you see instead?

The README refers to ubuntu:16.04 and this is old stuff that is not used, If you want to create a similar images then `build/build_java_test_image.sh` which uses Maven.

### Anything else?

_No response_

### Are you willing to submit a PR?

- [X] I'm willing to submit a PR!

Inspect the build/docker directory and its README, then search the repository for uses of its Dockerfile and the apachepulsar/pulsar-build image. Compare this with build/build_java_test_image.sh. Done means either the README clearly marks the directory obsolete or the unused directory is removed, with repository references accounted for.

Written by the indexing model from the issue text.