Fix stopping container with running xtail

Always exec shiny-server as container's main process and run `xtail` in background. So, both processes stop on SIGTERM as expected.

shiny-server.sh

@@ -4,13 +4,11 @@
 mkdir -p /var/log/shiny-server
 chown shiny.shiny /var/log/shiny-server
 
-if [ "$APPLICATION_LOGS_TO_STDOUT" = "false" ];
+if [ "$APPLICATION_LOGS_TO_STDOUT" != "false" ];
 then
-    exec shiny-server 2>&1
-else
-    # start shiny server in detached mode
-    exec shiny-server 2>&1 &
+    # push the "real" application logs to stdout with xtail in detached mode
+    exec xtail /var/log/shiny-server/ &
+fi
 
-    # push the "real" application logs to stdout with xtail
-    exec xtail /var/log/shiny-server/
-fi
+# start shiny server
+exec shiny-server 2>&1