ensure docker guest builddir is not tainted from a previous host build

[cosmicpsyop]

was able to repro'ed dockerfile error with jammy. what was discovered was the host directory was tainted from a build that used flatbuffers 2.0.8
alternatively, one could just manually do it.

make clean
docker build . -t cosmicpsyop/strfry -f ubuntu.Dockerfile
docker run -v $HOME/src/strfry/strfry-db:/app/strfry-db -v /home/xsited/src/forks/strfry/strfry.conf:/app/strfry.conf  -t cosmicpsyop/strfry   

testing: #86
#69

[hoytech]

Thanks for this!