Update Dockerfile

Author: LiveOverflow

ctf/Dockerfile

@@ -13,6 +13,6 @@ pip3 install pwntools keystone-engine unicorn capstone ropper && \
 mkdir tools && cd tools && \
 git clone https://github.com/JonathanSalwan/ROPgadget && \
 git clone https://github.com/radare/radare2 && cd radare2 && sys/install.sh && \
-cd .. && git clone https://github.com/pwndbg/pwndbg && cd pwndbg && checkout stable && ./setup.sh && \
+cd .. && git clone https://github.com/pwndbg/pwndbg && cd pwndbg && git checkout stable && ./setup.sh && \
 cd .. && git clone https://github.com/niklasb/libc-database && cd libc-database && ./get && \
 gem install one_gadget