Fix "ray: command not found Error" with Kuberay v0.3.0+

[Yicheng-Lu-llll]

Recent updates to Kuberay (versions beyond 0.3.0) have been causing a /bin/bash: ray: command not found error when users attempt to create rayjobs through Flyte.

Why does this result in a /bin/bash: ray: command not found error?

1. The Dockerfile adds /opt/venv to PATH using ENV PATH="${VENV}/bin:$PATH", rather than writing it into files like .bashrc. Thus, if a new shell is initiated and reads files like .bashrc first, $PATH may get overridden. The recent modifications to Kuberay initiate such a shell and thus override the PATH (see this PR for more details), causing the /opt/venv directory not to be included in the PATH.

How to resolve this?

This issue can be addressed by eliminating the redundant Python3 virtual environment. Given that the base image is python:3.8-slim-buster, there's no need to establish an additional Python virtual environment in the /opt/venv directory, nor is there a need to add /opt/venv to PATH.