Change docker label format. Should not support bracket chars

Hi,

I am trying to use this sidecar container in an architecture with Amazon EC2 Container Service (ECS). When I try to configure an ECS task definition with the label format `connect[8081]=stock-price.service.consul` it gives the following error message:

`Docker label key connect[8081] contains invalid characters, does not match pattern ^[a-z0-9-.]+$`

According to the docker spec the ECS service is correct in checking that the labels contain only lower-cased alphanumeric characters, dots and dashes (see: https://docs.docker.com/engine/userguide/labels-custom-metadata/). It seems however that docker engine is not strict in enforcing this.

Is there any chance this can be changed to support valid Docker tag as per the spec?