POC: Speed up compilation by freezing container during compilation.

[jonathanmetzman]

Instead of rebuilding the entire project every time we want to compile a single fuzz target, a better workflow is to build the project once and somehow compile the target against the already compiled project code.
This POC does that by interrupting building in when it detects it is compiling the fuzz target.
On detection it does the following:

1. Writes the command to /out/statefile TODO: write the cwd.
2. Commits the current container as "frozen" for use later. TODO: make this changeable.
3. Returns 1 so compilation stops. TODO: It would be better to exit the container.
   This step may be important to prevent clean up of the environment.

Then the frozen container can be used to compile fuzz targets against the project without recompiling the project in its entirety.

TODO:

1. Support this in oss-fuzz-gen
2. Install docker command line tool in base-builder (or use sneaky inheritance) because it must be used within the container.
3. Automate the compilation of the new fuzz target

[jonathanmetzman]

Command to test:

export PROJECT=libpng; python3 infra/helper.py build_image $PROJECT --no-pull && sudo rm -f build/out/$PROJECT/statefile.json  && docker run --rm --privileged --shm-size=2g --platform linux/amd64 -i -e FUZZING_ENGINE=libfuzzer -e SANITIZER=address -e ARCHITECTURE=x86_64 -e PROJECT_NAME=$PROJECT -e HELPER=True -e FUZZING_LANGUAGE=c++ -e CC="/usr/local/bin/clang-jcc" -e CXX="/usr/local/bin/clang++-jcc" -v $HOME/oss-fuzz/build/out/$PROJECT/:/out -v $HOME/oss-fuzz/build/work/$PROJECT:/work -v /var/run/docker.sock:/var/run/docker.sock -it gcr.io/oss-fuzz/$PROJECT "bash" "-c" "cp /work/clang*-jcc /usr/local/bin/ && compile"

[DonggeLiu]

IIRC, os.O_APPEND caused an invalid JSON format during our hackathon, is it the same case here?