Dockerfile: add curl to dockerfile for health check purposes

[tornadocontrib]

Please add CURL command line tool for health checking purpose on docker ( and it is also useful to test if the container is reachable to network )

[fjl]

We would like to keep the container image as minimal as possible. If you require another tool in the image, you will have to create your own image based on ours.

[tornadocontrib]

@fjl Yes I just found that it is better to use geth IPC console for the purpose

healthcheck:
      test: ["CMD-SHELL", "geth attach --exec 'net.peerCount' /geth/geth.ipc"]
      interval: 30s
      timeout: 5s
      retries: 10
      start_period: 20s