Allow to link SCION configuration files as bind mounts into the container instead of copying everything in build process

[martenwallewein]

The goal is to avoid rebuilding everything when e.g. changing log level. more information following.

[amdfxlucas]

the idea is basically to not bake the contents of '/etc/scion' (scion config files .toml|.json & secret keys ) into the docker images with 'COPY ..' directive but instead mount it as a shared volume in the docker-compose.yml file like:
volumes:
- type: bind
source: ./
target: /etc/scion

drawback is that this will probably won't work in swarm mode