Make container exit gracefully and with correct exit code

While fiddling around with multiple child processes, and giving them
some nice "exit 1" error codes, I noticed that the container would
always exit with exit code "0". It was not until I placed an
"exit 2" above the 'trap "kill 0" EXIT' row that I started getting
the expected exit codes again.

I dived a bit deeper into this, and realized that this thing with
properly handling kill signals is pretty tedious. Nevertheless,
after reading a lot of articles (linked at the bottom) I finally
managed to create something which seems to work as intended.

The first trap will catch the SIGINT (Ctrl+C) and the SIGTERM
(docker stop ...) singnals, and make so that both of these trigger
a normal "exit" command in the script. The second trap is special
for bash (the 'EXIT' at the end of the trap command), since this
allows you to trigger a "cleanup script" before  exiting the main
program. I made so that it will run the function "clean_exit" before
terminating with the correct exit code.

The "clean_exit" script just sends a SIGTERM signal to the PIDs of
our two child processes (if they exists and have started), which
gives them a chance to exit gacefully.

By implementing this we can now exit the container in a controlled
and graceful manner, while also having it report the correct exit
code depending on what happened.

[1]: http://veithen.io/2014/11/16/sigterm-propagation.html
[2]: https://www.linuxjournal.com/content/bash-trap-command
[3]: https://blog.codeship.com/trapping-signals-in-docker-containers/
[4]: https://www.ctl.io/developers/blog/post/gracefully-stopping-docker-containers/
[5]: https://unix.stackexchange.com/a/444676
[6]: https://blog.phusion.nl/2015/01/20/docker-and-the-pid-1-zombie-reaping-problem/
[7]: https://stackoverflow.com/a/35410993
[8]: https://unix.stackexchange.com/q/317492

src/scripts/entrypoint.sh

@@ -1,8 +1,21 @@
 #!/bin/bash
 
-# When we get killed, kill all our children (o.O)
-trap "exit" INT TERM
-trap "kill 0" EXIT
+# Helper function to gracefully shut down our child processes when we exit.
+clean_exit() {
+    for PID in $NGINX_PID $CERTBOT_LOOP_PID; do
+        if kill -0 $PID 2>/dev/null; then
+            kill -SIGTERM "$PID"
+            wait "$PID"
+        fi
+    done
+}
+
+# Make bash listen to the SIGTERM and SIGINT kill signals, and make them trigger
+# a normal "exit" command in this script. Then we tell bash to execute the
+# "clean_exit" function, seen above, in the case an "exit" command is triggered.
+# This is done to give the child processes a chance to exit gracefully.
+trap "exit" TERM INT
+trap "clean_exit" EXIT
 
 # Source "util.sh" so we can have our nice tools.
 . $(cd $(dirname $0); pwd)/util.sh