Add proxy configuration

`cfn submit` doesn't seem to work if a proxy is configured. Adding **use_config_proxy** to the _docker_build method in codegen.py shown below seems to resolve the issue.

```python
@classmethod
def _docker_build(cls, external_path):
cls._check_for_support_lib_sdist(external_path)

internal_path = PurePosixPath("/project")
command = " ".join(cls._make_pip_command(internal_path))
LOG.debug("command is '%s'", command)

volumes = {str(external_path): {"bind": str(internal_path), "mode": "rw"}}
image = f"lambci/lambda:build-{cls.RUNTIME}"
LOG.warning(
"Starting Docker build. This may take several minutes if the "
"image '%s' needs to be pulled first.",
image,
)
docker_client = docker.from_env()
try:
logs = docker_client.containers.run(
image=image,
command=command,
auto_remove=True,
volumes=volumes,
stream=True,
use_config_proxy=True #<=== use proxy configuration if specified in config.json
)
```