Benchmark framework docker flow should not run as root

While review feedback was in process for #337, Envoy landed a breaking change the docker images it pushes, as they no longer run as root by default. To resolve, this TODO was left in code comments:

```python
# As of https://github.com/envoyproxy/envoy/commit/e8a2d1e24dc9a0da5273442204ec3cdfad1e7ca8
# we need to have ENVOY_UID=0 in the environment, or this will break on docker runs, as Envoy
# will not be able to read the configuration files we stub here in docker runs.
```

Ideally we follow up with a change that would not make us need `ENVOY_UID=0`