[lib-injection] Set the pre built artifact that is used to build the image as its own stage

[pawelchcki]

• ADD docker commands should be avoided as they can invoke shell scripts. - COPY is more cache friendly

Example use after this change (not requires latest docker - esp buildx and buildkit - to be installed)

mkdir build/
touch build/dd-java-agent.jar
docker buildx build https://github.com/DataDog/dd-trace-java.git#pawel/better_lib_injection_dockerfile:lib-injection/src/main/docker --build-context java_agent=./build/

[pawelchcki]

Thanks for the quick review @devinsba 🙇 !