Support for using Docker daemon behind proxy

Currently, using Docker in Docker does not work behind a proxy when running the code build images locally. The Docker daemon requires a proxy to be set using `systemctl` which is not available on the image, so I am unable to reach out to the Docker registry to pull images when building a new image. Building artifacts work perfectly fine, but it would be nice if I could build a docker image within the Code Build image locally to help with testing, rather than testing that using the actual Code Build service.