Add user to docker group in Debian package (#847)

supplemental/debian/postinstall.sh

@@ -31,6 +31,12 @@ if ! getent passwd "$SERVICE_USER" >/dev/null; then
 		--gecos "System user for $SERVICE"
 fi
 
+# Enable docker
+if ! getent group docker | grep -q "$SERVICE_USER"; then
+	echo "Adding $SERVICE_USER to docker group"
+	usermod -aG docker "$SERVICE_USER"
+fi
+
 # Create config file if it doesn't already exist
 if [ ! -f "$CONFIG_FILE" ]; then
 	touch "$CONFIG_FILE"