Add rcon client (#550)

* Add rcon client to container

* Explain rcon command

* Remove test container name

* Apply suggestions

Co-authored-by: Florian Kinder <florian.kinder@fankserver.com>

* Add example docker-compose file

* Clarify return code

* Switch to pre-update

* Update docker-compose.yml

Co-authored-by: Florian Kinder <florian.kinder@fankserver.com>

* Allow build support for build (/* is only possible in buildx)

* Added version information in README.md

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Co-authored-by: Florian Kinder <florian.kinder@fankserver.com>

docker/rcon/Makefile

@@ -0,0 +1,13 @@
+# Optimization
+OPT = -O3 -flto
+TARGET = rcon
+
+CC = gcc
+CFLAGS = -std=c17 -Wall -Wextra -pedantic $(OPT)
+REMOVE = rm -f
+
+all:
+	$(CC) $(CFLAGS) main.c -o $(TARGET)
+
+clean:
+	$(REMOVE) $(TARGET)