WIP: Slim docker
@MarcNicolas & @JulienPeloton : Backend build in container seems right, but execution fails:
File "/usr/lib/python3.6/site-packages/flask/cli.py", line 235, in locate_app
__import__(module_name)
File "/app/pspa.py", line 5, in <module>
from backend import element, pyelements
ImportError: Error relocating /app/backend/pyelements.cpython-36m-x86_64-linux-gnu.so: fodo_machine: symbol not found