docker: add doc

docker/Dockerfile

@@ -70,6 +70,12 @@ HEALTHCHECK \
 	--retries=3 \
 	CMD [ "/opt/adguardhome/scripts/healthcheck.sh" ]
 
+# It seems that the healthckech script sometimes spawns zombie processes, so we
+# need a way to handle them, since AdGuard Home doesn't know how to keep track
+# of the processes delegated to it by the OS.  Use tini as entry point because
+# it needs the PID=1 to be the default parent for orphaned processes.
+#
+# See https://github.com/adguardTeam/adGuardHome/issues/3290.
 ENTRYPOINT [ "/sbin/tini", "--" ]
 
 CMD [ \