`vpn` environment : misc containers refactor

This merge requests covers pieces of #138 (closed) "VPN/container env better packaging".

• change config.env VPN configuration files handling
  • goal: no config.env used in docker-compose.yml file, so that we can docker build when the config.env don't exist
  • goal: no config.env under git tracking, so that they don't git diff when running
  • principle: config.env handling does not rely anymore on the env_file tag of docker-compose.yml (aka passing env variable to container). We now source the config.env file when needed aka (1) in the entrypoint.sh script (2) in the .bashrc of the container's account (note: this will only be done when using bash, but anyway sourcing .bashrc is optional, it is done for convenience so that VPN env is already set when connecting to container)
  • changes for user: see README_tempo.md aka
    • template for vpnserver config file now in config.env.template so start with a cp config.env.template config.env
    • config.env syntax changed from VAR=value to export VAR=value
• misc: set easy to understand hostname in containers (eg: node containers have hostname fedbiomed-vpn-node)
• misc: add EXPOSE (informtational tag) to dockerfile to document exported ports (good practice)