Security review fixes

[ArseniiPetrovich]

This PR is a cumulative update that fixes some of the critical issues that were found during security audit.

• CPU and memory limits for containers
• ReadMe fixed at https://github.com/poanetwork/token-bridge#docker.
  As for https://github.com/poanetwork/token-bridge#rollback-the-last-processed-block-in-redis and https://github.com/poanetwork/token-bridge#native-to-erc20-mode-testing. I might be mistaking, but I didn't find anything to fix. We still have the bridge service which can be used as the part of the commands at the ReadMe.
• VALIDATOR_ADDRESS is removed from where it is not needed. However, I'm not sure if Add support ERC20 to Native #85 is correct since VALIDATOR_ADDRESS is still needed by some of the containers (where VALIDATOR_ADDRESS_PRIVATE_KEY is not specified)
• .dockerignore updated
• Networks separated at the containers

References:
#116
#113