Added support for supplying supplementary groups of the X application.

README.md

@@ -137,6 +137,9 @@ to the `docker run` command.
 - **GROUP_ID**
   ID of the group the X application run as.  Default is `1000`.  See
   [User/Group IDs](#usergroup-ids) to better understand when this should be set.
+- **SUP_GROUP_IDS**
+  Comma-separated list of supplementary group IDs of the X application.  By
+  default, this variable is  not set.
 - **APP_NICENESS**
   Priority at which the X application should run.  A niceness value of −20 is
   the highest priority and 19 is the lowest priority.  By default, niceness is

rootfs/etc/services.d/guiapp/run

@@ -11,7 +11,11 @@ done
 
 echo "[services.d] guiapp: starting "${APP_NAME:-X application}"..."
 if [ -e /startapp.sh ]; then 
-    exec 2>&1 $APP_NICE_CMD s6-setuidgid $USER_ID:$GROUP_ID /startapp.sh
+    if [ "${SUP_GROUP_IDS:-UNSET}" != "UNSET" ]; then
+        exec 2>&1 $APP_NICE_CMD s6-applyuidgid -u $USER_ID -g $GROUP_ID -G $SUP_GROUP_IDS /startapp.sh
+    else
+        exec 2>&1 $APP_NICE_CMD s6-setuidgid $USER_ID:$GROUP_ID /startapp.sh
+    fi
 else
     echo ""
     echo "/!\\ No application to start: /startapp.sh is missing. /!\\"