playground: Revert Dockerfile to installing the entire VANADIUM_ROOT.
playground: Revert Dockerfile to installing the entire VANADIUM_ROOT.

Since VANADIUM_ROOT inside the Docker image is owned by the root user,
installing only playground prerequisites wasn't sufficient since the
builder had insufficient privileges to build any additional libraries
required by user code.

Change-Id: Ie6ecd8e7dd127dc1e12b24f9529401451c5c84ac

vanadium/playground,vanadium/playground,vanadium/playground,vanadium/playground