Add rcon client to container

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)