Scape $USER in Dockerfile to avoid bash expansion

Keep Dockerfile variables in placce. Should help with the local
reproducibility.

Signed-off-by: Jose Luis Rivero <jrivero@osrfoundation.org>

jenkins-scripts/docker/lib/docker_generate_dockerfile.bash

@@ -456,8 +456,8 @@ RUN echo '%sudo ALL=(ALL) NOPASSWD:ALL' >> /etc/sudoers
 RUN chown -R \$USER:\$USER /home/\$USER
 
 # permit access to USER variable inside docker
-ENV USER ${USER}
-USER $USER
+ENV USER \$USER
+USER \$USER
 # Must use sudo where necessary from this point on
 DELIM_DOCKER_USER