fix: change app name to gnome

Author: inureyes

ubuntu/Dockerfile.18.04.gnome.amd64

@@ -101,7 +101,7 @@ LABEL ai.backend.kernelspec="1" \
       ai.backend.resource.preferred.shmem="512m" \
       ai.backend.base-distro="ubuntu18.04" \
       ai.backend.runtime-type="app" \
-      ai.backend.service-ports="xfce:http:80,vnc:http:5900"
+      ai.backend.service-ports="gnome:http:80,vnc:http:5900"
 
 COPY ./service-defs /etc/backend.ai/service-defs
 COPY policy.yml /etc/backend.ai/jail/policy.yml