Dockerfile: add gdb

tools/docker/Dockerfile

@@ -5,7 +5,7 @@ RUN apt-get update && \
   apt-get install -y --no-install-recommends \
   build-essential doxygen git wget unzip python-serial python-pip \
   default-jdk ant srecord iputils-tracepath rlwrap \
-  mosquitto mosquitto-clients \
+  mosquitto mosquitto-clients gdb \
   && apt-get clean
 
 # Install ARM toolchain