scripts: use simple form of `--env` parameter to specify `DISPLAY`

Makefile

@@ -23,7 +23,7 @@ google-chrome tor-browser chromium-browser firefox bash:
 	@docker run -it --rm --cap-add=SYS_ADMIN \
 		--env="USER_UID=$(shell id -u)" \
 		--env="USER_GID=$(shell id -g)" \
-		--env="DISPLAY=${DISPLAY}" \
+		--env="DISPLAY" \
 		--volume=/tmp/.X11-unix:/tmp/.X11-unix \
 		--volume=/run/user/$(shell id -u)/pulse:/run/pulse \
 		${USER}/browser-box:latest $@

scripts/browser-box

@@ -29,7 +29,7 @@ else
     --cap-add=SYS_ADMIN \
     --env="USER_UID=${USER_UID}" \
     --env="USER_GID=${USER_GID}" \
-    --env="DISPLAY=${DISPLAY}" \
+    --env="DISPLAY" \
     --volume=/tmp/.X11-unix:/tmp/.X11-unix \
     --volume=/run/user/${USER_UID}/pulse:/run/pulse \
     sameersbn/browser-box:latest $(basename $0) $@ >/dev/null