Fixing the location of build.sh gets rid of ENTRYPOINT. Which opens a possibility to implement different commands like:
````bash
docker run -ti expat compile
docker run -ti expat run <fuzzer_name> <input_data>
````
and keep the knowledge of build script location.
This is a breaking change.
We may want to reintroduce it to make the build process prettier
and easier to debug; but we should do it for all the libraries we're
fuzzing, not just this one.