Revert 1502 - Switching docker user to lighthouse (#1578)

## Issue Addressed

The lighthouse user has recently changed to `lighthouse` from root. 

This requires uses to change ownership of their current docker mounted volumes and the upgrade path is non-trivial. 
This reverts #1502 and we will include it in a major release in the future.

## Proposed Changes

N/A

## Additional Info

N/A

Dockerfile

@@ -12,7 +12,5 @@ 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

book/src/docker.md

@@ -53,7 +53,7 @@ $ docker run lighthouse:local lighthouse --help
 You can run a Docker beacon node with the following command:
 
 ```bash
-$ docker run -p 9000:9000 -p 127.0.0.1:5052:5052 -v $HOME/.lighthouse:/home/lighthouse/.lighthouse sigp/lighthouse lighthouse --testnet medalla beacon --http --http-address 0.0.0.0
+$ docker run -p 9000:9000 -p 127.0.0.1:5052:5052 -v $HOME/.lighthouse:/root/.lighthouse sigp/lighthouse lighthouse --testnet medalla beacon --http --http-address 0.0.0.0
 ```
 
 > To join the altona testnet, use --testnet altona instead.
@@ -62,7 +62,7 @@ $ docker run -p 9000:9000 -p 127.0.0.1:5052:5052 -v $HOME/.lighthouse:/home/ligh
 
 ### Volumes
 
-Lighthouse uses the `/home/lighthouse/.lighthouse` directory inside the Docker image to
+Lighthouse uses the `/root/.lighthouse` directory inside the Docker image to
 store the configuration, database and validator keys. Users will generally want
 to create a bind-mount volume to ensure this directory persists between `docker
 run` commands.
@@ -71,7 +71,7 @@ The following example runs a beacon node with the data directory
 mapped to the users home directory:
 
 ```bash
-$ docker run -v $HOME/.lighthouse:/home/lighthouse/.lighthouse sigp/lighthouse lighthouse beacon
+$ docker run -v $HOME/.lighthouse:/root/.lighthouse sigp/lighthouse lighthouse beacon
 ```
 
 ### Ports