docs: Update Docker docs and docker-compose, mention group-add (#4237)

docker-compose.yaml

@@ -16,6 +16,12 @@ services:
       # Alternatively, you can enable CODER_TUNNEL for
       # proof-of-concept deployments.
       CODER_TUNNEL: "${CODER_TUNNEL:-false}"
+    # If the coder user does not have write permissions on
+    # the docker socket, you can uncomment the following
+    # lines and set the group ID to one that has write
+    # permissions on the docker socket.
+    #group_add:
+    #  - "998" # docker group on host
     volumes:
       - /var/run/docker.sock:/var/run/docker.sock
     depends_on: