FR: Added expanding environment variables for allow* methods

[wollomatic]

An interesting FR by @AlexSat (see PR #48)

Allow to place environment variable into allow* variables content for future expanding when starting to grant permissions for target container only.

Use case(for docker ocmpose): set environments
SP_ALLOW_POST=/v1\.\d{1,2}/(exec|containers)/$$CONTAINER_NAME/(start|stop|exec)
CONTAINER_NAME=xxx

More complex example:

#cat exts.yaml
services:
  docker-proxy:
    image: wollomatic/socket-proxy:1
    restart: unless-stopped
    read_only: true
    mem_limit: 64M
    hostname: docker_proxy
    security_opt:
      - no-new-privileges
    user: 65534:${DOCKER_GROUP_ID}
    volumes:
      - /var/run/docker.sock:/var/run/docker.sock:ro
    environment:
      - SP_LISTENIP=0.0.0.0
      - SP_LOGLEVEL=INFO
      - SP_PROXYPORT=${DOCKER_PROXY_PORT}
    networks:
      - docker_proxy_net
  docker-proxy-start-stop:
    extends: docker-proxy
    environment:
      - SP_ALLOW_POST=/v1\.\d{1,2}/containers/$$TARGET_CONTAINER/(start|stop)
  docker-proxy-start-stop-exec:
    extends: docker-proxy
    environment:
      - SP_ALLOW_POST=/v1\.\d{1,2}/(exec|containers)/$$TARGET_CONTAINER/(start|stop|exec)
      - SP_ALLOW_GET=/v1\.\d{1,2}/(exec|containers)/.*?/(json)

#cat compose.yml
services:
  ...
  docker_socket:
    extends:
      file: exts.yml
      service: docker-proxy-start-stop
    environment:
      - SP_ALLOWFROM=my_app_backup
      - TARGET_CONTAINER=my_app

[wollomatic]

see #48 (comment)