Expand build instructions

[elharo]

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