Fix loss of worker nodes on restart (Fixes #11)

[pendor]

The docker entry point is too aggressive in overriding the current server list each time the container restarts. This results in loss of any added worker nodes on each restart. PR bumps to latest upstream Cronicle and modifies the entry point to do nothing if container has already been configured.

Fixes #11

Note that a fully working environment may also require adding a local volume overlay for the conf directory. IE:

volumes:
      - /etc/localtime:/etc/localtime:ro
      - /etc/timezone:/etc/timezone:ro
      - ./data/data:/opt/cronicle/data
      - ./data/logs:/opt/cronicle/logs
      - ./data/conf:/opt/cronicle/conf
      - ./data/plugins:/opt/cronicle/plugins