fix(uv-env): copy project to writable dir before install in container

[jinbai340997]

The UvRuntimeEnv mounts the host project directory as read-only (:ro)
into the container. However, setuptools requires writing egg-info
metadata to the source directory during uv pip install, which fails
on a read-only filesystem. Fix by copying the project to a writable
temporary directory (/tmp/rock-build) before running the install.
Also add .rayignore to exclude .venv, .git, pycache and other
non-essential files from Ray working_dir uploads. Fixes #856

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