Expand build instructions

### Affected version

HEAD

### Bug description

The usual git clone / mvn compile / mvn test / etc. are insufficient so more detailed docs are warranted.

```
[ERROR] Failed to execute goal org.apache.maven.plugins:maven-compiler-plugin:3.15.0:compile (compile-java9) on project docker-exe: Compilation failure
[ERROR] /Users/elharo/maven-executor/providers/docker-exe/src/main/java9/module-info.java:[23,41] module not found: org.apache.maven.executor
```

Start by reproducing the documented git clone, mvn compile, and mvn test flow, then investigate the Java 9 module-info.java error showing that org.apache.maven.executor is missing. Document the additional setup or commands required for a successful build and test run, including the relevant prerequisites; done means a newcomer can follow the instructions without encountering this failure.

Written by the indexing model from the issue text.