message: Update Docker README.md to reflect symbol argument
diff: @@ -26,6 +26,12 @@ The above will build off the most recent commit to the master branch by default.
  docker build -t eosio/eos:dawn-v4.0.0 --build-arg branch=dawn-v4.0.0 .
  ```
  +By default, the symbol in eosio.system is set to SYS. You can override this using the symbol argument while building the docker image.
  +
  +```bash
  +docker built -t eosio/eos --build-arg symbol=<symbol> .
  +```
  +
  ## Start nodeos docker container only
  ```bash
  