Creates a new lighthouse user and makes it the default user to be use… (#1502)

…d in the Docker image

## Issue Addressed
https://github.com/sigp/lighthouse/issues/1459

## Proposed Changes

- Create new `lighthouse` user and group in Docker container
- Set user as the default user

Dockerfile

@@ -12,5 +12,7 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
   ca-certificates \
   && apt-get clean \
   && rm -rf /var/lib/apt/lists/*
+RUN groupadd -r lighthouse && useradd --no-log-init -r -g lighthouse lighthouse 
+USER lighthouse
 COPY --from=builder /usr/local/cargo/bin/lighthouse /usr/local/bin/lighthouse
 COPY --from=builder /usr/local/cargo/bin/lcli /usr/local/bin/lcli